Looking to get a 2" Revolver as a back up CCW. Trying to decide between a 38 Special or a 9mm in such a short barrel configuration. Assuming equal reliability... what offers the best ballistics in such a short barrel? Would it be better to have a heavier/slower bullet or a faster/lighter bullet?

Good question, one that I have pondered as well.
I actually like a revolver and the technical improvements have been concentrated on the 9mm for a while.
With that in mind and the incredible selection of 9mm rounds, I'd look at the 9mm variant for sure.

I may be wrong but I'd guess the 9mm looses less in the 2" barrel than the 38spl.
9mm is everywhere, it has become America's 38spl.
Either way I'd like to see the new Kimber revolver in 9mm.

This is a complicated issue and could generate "discussions".  I hand load and test all of my commercial and personal loads through a chronograph.  For years I was a fan of the mid weight to lightweight bullets and higher speeds due to kinetic energy.  But I have changed my opinion to heavier bullets due to more recent published gelatin tests and better bullet designs.  Here is a great read, for example:  http://www.ar15.com/ammo/project/Self_Defense_Ammo_FAQ/index.htm.  I believe ammunition/bullet choice is far more important than caliber.

"Heavy" is relative for each caliber.  However, when speaking of short barrels of any caliber or weapon type (handgun and rifle), the concern is more associated with bullet performance as a result of its design parameters and velocity.  2" barrels lose a lot of velocity.  All things else being equal with expanding bullets, a lighter faster bullet will penetrate less than a heavier slower bullet since the faster bullet most likely will expand to a greater diameter and penetrate less.  Mass makes a large difference.

More to your point with a relative apples to apples comparison using a Kahr PM9, 3" barrel including chamber and a Ruger SP101, 2.25" barrel not including chamber which are functionally very close.  Without getting into minutia detail, they are relatively equal in Power Factor, which includes velocity and bullet weight.  Today's ammunition is pretty good.  Most good ammunition have a PF of 138 - 142 in either of these guns.

That said, my SP101 with Buffalo Bore (#19G) 125 grain JHP has a PF of 163 and their .38 Special (#20A) 158 grain LSWC HP has a PF of 161.  Very stout ammunition in a small revolver.  If I did a gelatin test I believe I would see much greater penetration from the heavier bullet.  I do not have Buffalo Bore's ammunition in 9MM but believe it would have similar power factors.

A couple years ago I switched from auto loaders for CCW to revolvers with all ammunition being flash suppressed, if possible.  I really miss my awesome Glock G36 but now carry a Ruger LCR/357, 1.8" barrel, using .38 Special +p ammunition from Buffalo Bore or my winter gun of a Taurus .44 Special using Buffalo Bore ammunition.  I highly prefer Gold Dot bullets and will be getting Speer 135 grains as soon as I can find any for the LCR.  Speer and Buffalo Bore are my favorite ammunition suppliers in any caliber with Buffalo Bore being the most powerful I have tested from any of the major OEMs.

Unless you are heavily invested in 9mm (ie piled deep and/or reloading it) a 9mm revolver does not make a lot sense.  At least not with current offerings in 9mm revolvers.  No manufacture makes a dedicated 9mm sized revolver frame.  They just take their 357 Magnum revolver and chamber the cylinder for 9mm.  You have this relatively short cartridge in a unnecessarily long cylinder.  9mm OAL being 1.169 vs 357 Mag at 1.59.

If they actually made a 9mm length frame similar to the old S&W I-frame then you might have something unique and useful.  In theory you could create an I-frame that would have the same overall size as a J-frame but with a ~3/8 longer barrel and due to the shorter frame and cylinder a bit less mass and bulk.  But there is not enough market to make a dedicated 9mm frame a viable product (except Korth) so you end up with 9mm in a 357 Mag.  357 Magnum is a lot more flexible chambering.  Bullets as heavy as 180 or more can be utilized and you can always shoot 38 Special, 38 Long Colt or even 38 Short Colt. in it.

If you really want to make it flexible, buy a 357 mag revolver and send it to TK Custom and have it converted to 9mm on moonclips.  Now you can shoot all three cartridges (9mm/38/357) in the same gun.  Although TK custom does not recommend +P or really hot 357 mag in their converted revolvers.

38 spl. The 9mm has quite a jump in a cylinder. In 38, wadcutter, 135gr or 125gr Gold Dot or 110gr Critical Defense get the job done, along with the 110gr Barnes bullet.

I did the same thing recently, and ended up w/ a 9mm LCR.  The ballistics surprised me, and I like the idea of ammo commonality with my primary.
Something I didn't expect is that I really like the moonclips.  

Some info on why the ballistics are as good as they are:
9MM LCR

Lol! The 9mm Korth Sky Hawk is literally what prompted me to ask this. With the new announcement from Colt for a 38 Special +P Cobra, I'm debating between the 38 Special and a 9mm for a 2" snubby revolver as a BUG or maybe even an ankle gun.

If you are set on a revolver, .357 magnum.
.38 SP is rather anemic especially by today's standards. It was good when it came out but 100+ years of innovations have come along.
9MM is pretty good in a SB pistol and the ammunition selection is very good.

Here's an amusing challenge, load up a 642/442 with 38 Special +P and a 340 with self defense 357 Mag loads.  Then two shooters race to get off 6 rounds 38 Special vs 357 Mag.  The looser buys the beer when the feeling returns to his hand so he get the cash out of his wallet.  The 38 Special shooter is going to get a lot of free beer.

Very few people can shoot a light weight (<20oz loaded) snub nose 357 Magnum well enough.  Go over to the revolver section and look at some of the recent threads and the number of guys that buy a 357 Mag snub nose Airweight revolvers then after a few range sessions realize that they can't handle 357 Mag in such a light gun and end up carrying 38 Special +P in it.  38 Special +P is, with correct ammunition selection, a very effective caliber for self defense.  357 Magnum, all be it a lot more cartridge from a ballistics point of view, is not statically that much more effective in self defense situations when used in a snub nose revolver, especially in light of the difficulty the recoil causes in shooting them well.  Not to mention 38 Special +P snub nose revolvers are much cheaper than 357 Mag version.  That's a lot of practice ammo.

I hear you.  I have a S&W M&P 340, 11.4 oz, Scandium.  A beautiful piece of equipment that is meant to carry a lot and shoot just a little.  I had this to carry when nothing else will do due to clothing types and activities.  I only purchase .357 Magnum versions to ensure I have a better chance of ammunition availability and a gun that lasts longer under heavy loads.  It is loaded with .38 Special +P from Buffalo Bore and is not a pleasant piece to shoot more than a cylinder or two while practicing.  And in .357, maybe one round- not more than two or three.  A large part of this experience is the tiny grip.

My preferred warm weather carry is a Ruger LCR .357 that weighs 17 ounces and retails for $90 more (street price of $65 more) than the .38 Special version and is less than half the price of the 340.  Had the LCR been available when the 340 was, I never would have purchased the 340.  The LCR handles any load much better than the 340 and is nearly as concealable.  The .38 Special version weighs ~13.5 ounces so will be a handful with +P ammunition, where the .357 version tames it down more due to being a bit heavier.  .357 ammunition is more shootable in this also.  However, .357 ammunition out of a short barreled revolver is terribly loud, and as you mentioned, followup shots are much slower and less accurate.  YMMV.

I have a Ruger LCR 38 SPL +P rated revolver and a Ruger LCR 9mm.  Neither are pleasant to shoot more than a few cylinders, but by far, the 9mm with 124 gr +P ammo has the harshest recoil impulse over the 38 SPL with +P ammo.  Both these LCR's are great as BUGs, very concealable, but I'm much more proficient with my semi-auto Shield 9, which is also very concealable.  It would be tough to shoot 357 Magnum from the LCR platform well.  Yes... YMMV.

.38Special has came a long way in regards to ammo/loading developments especially within the last 10 years. The Gold Dot 135gr +P is one heck of a good performer from 2" barrels. Hornady 158gr XTP's in standard pressure have demonstrated to be devastating in some recent gel test I've seen. Hell, the 125gr Gold Dots are pretty nasty as well even from a 2" snubby... just to name a few of the current and effective loads being offered currently. All of the aforementioned penetrate past 12 inches thru 4 layers of denim in 10% ordinance gel and all expand to over .50 caliber at a minimum... Then you get into boutique loadings from Buffalo Bore using the venerable "Keith Bullet" design in 158grains; and you have bullets that surpass even some well regarded 9mm self defense loads.

I carry a CZ P-01 (9mm) daily and when going into the shittiest parts of Atlanta I have no problem carrying anyone of the aforementioned loads in my .38Spec "Chiefs Special."

Bullet design greatly favors the .38 special in snub nose handguns.  The original FBI load, .38 special lead swchp, almost always expands from a short barrel and has a great track record for effectiveness.  It is a bullet design that is incapable of being duplicated in an auto cartridge like the 9mm.  

I still don't think there is a better load for the 38 special than the old FBI load, I'd take it over the new speer gold dot anytime.

I personally prefer the Remington variety (marked as 158 gr LHP) as the lead is slightly softer than the Fed and Winchester offers, giving better expansion.

This is what you want!

Just my opinion, but buying a 9mm revolver makes about as much sense as buying a .38 Special semi-auto.

That aside, from a paper ballistics standpoint the 9mm has it all over the .38 Special with most bullets, but when bullet weights exceed 150 grs. or so the .38 pulls ahead as it can be loaded with bullets up to 200 grs. in weight. I personally prefer bullets on the heavy side. I loaded 153 gr. cast SWCHP's for my wife's snubbie, and have tested bullets as heavy as 173 grs.

Yes, the old venerable 158gr soft lead hollow point FBI load punches deep and hits hard... And from my experience so does the more modern 125grain Gold Dot; and from other's experience/test it's looking like the 135gr Gold Dot+P's are popping cherries like Prom Night with .38Special 2" barrled snubs... I have a pair of S&W M-36 "J" Frames that I carry the Buffalo Bore 158gr duplicated FBI loads for primary (chamber loaded), and the 125gr Gold Dots for reloads. The jacketed Gold Dots are easier for me to load in a hurry from speed strips and/or than fully exposed lead bullets. YMMV...

For BUG purpose .38 is fine.  
My 442 with +P's works great for a BUG.

From Ammo FAQ "More information from Doctor Roberts: "With few exceptions, such as the Speer 135 gr +P JHP and Barnes XPB, the vast majority of .38 Sp JHP's fail to expand when fired from 2" barrels in the 4 layer denim test.  [...]   There is no reason to go with .357 mag in a J-frame, as the significantly larger muzzle blast and flash, and harsher recoil of the .357 Magnum does not result in substantially improved terminal performance compared to the more controllable .38 Special bullets when fired from 2” barrels

This may be true, and I have hear it repeated frequently on the internet but it still does not sit well with me.  From a snubby, 38 Special +P usually fails to perform but 357 Magnum it too much power, recoil, muzzle blast and yet somehow does not perform any better at the terminal end.  Smells like BS or incomplete research.

It definately performs better, gel tests on youtube confirm that.  Though the increase in blast and recoil is substantial.  I wouldn't put too much blind faith into what that so called Dr. says.  Always be wary of gurus, esp when so many people put so much blind faith behind them like ARF.

Here are some real chrono data out of a Ruger SP101, 2.25" barrel, which is my 38 Special/357 Magnum test vehicle, and a CED M2 chrono @ 10' using factory ammunition.  I also have hand load data that exceeds most factory ammunition (except Buffalo Bore) by a significant margin, but these are out of the scope of this discussion.  In my opinion, ammunition selection- in particular bullet type, velocity and suppressed powder usage, is far more important than typical self defense calibers/barrel lengths of .38 Special and greater.  Even for .45 auto.  Although .45 auto largely exceeds the data noted below.  And a shooter's accuracy trumps all else in most cases.

Buffalo Bore .357 (#19G) tactical short barrel, 125 grain JHP, avg. velocity= 1301.8, PF= 163
Buffalo Bore 38 Sp +P (#20A), 158 grain LSWC HP, avg. velocity= 1018.6, PF=161
Fiochi .357  (#357C), 125 grain SJSP, avg. velocity= 1261.2, PF= 158 
CCI Blazer 357 158 grain JHP, avg. velocity= 897.8, PF= 142
Buffalo Bore 38 Sp +P, 125 grain GDHP (#20B), avg. velocity= 1088.8, PF= 136

And a comparison with a Kahr PM9, 3" barrel, which is my 9MM test vehicle:

Double Tap 9MM, 147 grain HP, avg. velocity= 962.8, PF= 142
Double Tap 9MM, 124 grain Gold Dot HP, avg. velocity= 1132.7, PF=140
Corbon 9MM +P, 115 grain JHP, avg. velocity= 1209.3, PF= 139
Winchester WB 9MM, 115 grain, FMJ, avg. velocity= 1021.9, PF=118

And a comparison with a Glock 27, 3.5" barrel, which is my standard .40 S&W test vehicle:

Corbon 135 grain, JHP, avg. velocity= 1195.3, PF= 159
Black Hills 155 grain GDHP, avg. velocity= 1024.2, PF= 159
Federal AE 155 grain, FMJ, avg. velocity= 999.5, PF= 155

I have a wealth of chrono data for most of my firearms and test through the chrono everything I shoot, including a very large variety of hand loads.  From anecdotal evidence, I believe the Speer GD 135 grain is one of the best short barrel .38 Special loads, have a bunch stocked and will test when I get caught up with building a new shooting range, among other things.  Speer Gold Dots are my bullets of choice in any caliber and I have hand loaded them also.

My carry piece used to be a Glock 36/.45 auto but is now a Ruger LCR in 357 mag using Buffalo Bore .38 SP/GDHP in the warmer months or a Taurus .44 Special (similar to a .40 S&W in computed power) in cooler months.

Nice data set.  That said PF is only good for comparing recoil in similar weight guns.  If your going to throw one number out there for terminal performance then kinetic energy is probably better.  KE with bullet weight and caliber tell a fairly complete story.  That said you can get to KE from PF if the bullet weight is known.

From a 2" barrel, .357mag and .38spl+P offer virtually identical ballistics.

Despite what some say, a .38 is fine. It has planted many people over the years. Speer Gold Dot 135gr .38 +P and Winchester PDX1 130gr .38 +P are quite impressive and would be my recommendations for a carry load. There is no question that a .357 Magnum is an effective round, but you can also do the job without the excessive recoil, blast, and flash.

BS!  357 Magnum produce significantly more velocity than 38 Special when compared pushing the same bullet weight in the same barrel length no matter what that barrel length that is.  Whether or not the terminal effects, from a short barrel, in a social situation, are better is a subject for much debate.  But there is not doubt that 357 Magnum loaded close to SAAMI max pressure of 35,000psi has significantly better exterior ballistics than 38 special +P at only 20,000psi.

The quote I posted above is from a very well renowned expert in terminal ballistics.  He thinks there is a serious doubt the .357 has better ballistics than .38.  I'm going to go with his opinion over that of random internet personas.

The .357 does have better performance then the .38, substantially so. They fire the same bullets, but the .357 fires them much faster.

You don't need "renowned expert in terminal ballistics" to read a chronograph of two identical projectiles, and surmise that the one with 50% more energy is preferable.

125gr +p .38 Special Gold Dot from a LCR: 900 fps / 225 ftlbs.
125gr .357 short barrel Gold Dot from J frame: 1,109fps / 341 ft/lbs

And this is where, getting back on topic, the 9mm revolver is so awesome:
Speer 124gr +p 9mm LCR: 1,099 fps

So, within 10fps of the .357, but with much less blast and flash, and far more available ammo.

I just wish S&W would make a Scandium 9mm J-Frame.

You are NOT getting 1,100 FPS out of a 2" barrel revolver w/ cylinder gap from ANY 9mm load.  I get 1,100 fps from a 4" barrel auto pistol, and you think you can get it from a 2" cylinder gap revolver?  

This was literally covered in a review of the LCR 9mm earlier in the thread:

Ruger LCR 9mm - faster then you think.

The reason is that revolver barrels are measured differently then a auto pistol; the Ruger LCR 9mm has an equivalent length of bullet travel to a 3.5" Auto pistol.

Lastly, properly loaded 9mm 124gr +p is going 1200 fps from a 4" barrel.

Your read comprehension is lacking. Get a chronograph and your favorite 357 Magnum snub nose revolver and go to the range.  Fire 357 Magnum rounds loaded to max SAAMI pressure for 357 Magnum and compare that to the same bullet loaded in a 38 Special loaded to +P max SAAMI pressure.  The 357 Magnum load will ALWAYS be faster.  357 Magnum ALWAYS has better internal and external ballistics compared to 38 Special +P  regardless of what barrel length we are comparing the two cartridges in.  That was the point of my previous post.  I believe your "well renowned expert" acknowledges that also.

Now the effectiveness of the Terminal ballistics of the two cartridges fired from a Snub Nose has room for a debate.  Does the additional velocity achieved by 357 Magnum in a Snub Nose over what is achieved by 38 Special +P make it's terminal ballistic performance better?  Does any benefit offset the additional recoil, noise and muzzle blast of 357 Mag over 38 Spl?  There is room for doubt, but there is also room to doubt your "expert".

If it makes you feel any better I carry a 38 Special +P not a 357 Mag Snub Nose.

Agree with this. I carry a vintage 1977' manufactured S&W M36 loaded with Buffalo Bore's tried and true ol' "FBI" load, soft cast, deep hollow point 158grain pills. They push +P velocities but are standard pressure and are safe to use in all .38Special pistols. I also like Gold Dot's 125 grain and Hornady's 158 XTP's, standard pressure as well in a pinch. All hit hard and deep.

I have carried j frames for many years as my go everywhere gun. The Glock 43 changed all that. Easy to carry in a front pocket in the summer.

Deal breaker for me is the ease of reload in my Glock vs my  j frame.

Ballistics favor the Glock as well

I just chronographed Speer 135 grain Gold Dot and wanted to add to this discussion.  Gold Dots are my preferred bullet in most calibers.  I used to hand load them in my pistols before switching to OEM self defense ammunition for the benefit of flash suppressed powders.

Following are chronograph results of .357/38 self defense type bullets using a Ruger SP101 with 2.25" barrel, which is my standard .357/.38 test device.  I am disappointed with the chronograph performance of the Speer 135 grain and will stick with Gold Dots and flash suppressed powder from Buffalo Bore for my Ruger LCR 357.

                                                                                                                 Avg. Vel    SD      Power Factor    Kinetic Energy in ft. pounds

11/16/2009  Buffalo Bore .38 Special +P. Item 20A  158 grain    LSWC HP         1018.6      16.6    160.9              363.9

11/16/2009  Buffalo Bore .38 Special +P.   Item 20B  125 Speer Gold Dot JHP    1088.8      9.9      136.1             329.0

3/24/2010    Double Tap   125 grain    .38 Special Gold Dot HP +P                    966.7        Unk     120.8             259.3

3/24/2010    Buffalo Bore (Item 20E)    125 grain .38 Special Gold Dot HP +P    939.5        5.4      117.4             244.9

5/16/2017    Speer .38 Special +P   135 grain Gold Dot HP                               864.3        17.5    116.7             223.9

2/29/2016    Magtec .38 Special +P (#BS1008 L-49) 125 grain    SJHP              865.3        11.7    108.2             207.8

2/29/2016    Remington UMC .38 Special +P (#4770035640)   125 grain SJHP   855           13.5    106.9            202.9