I'm looking to buy a new XD for IDPA, maybe some IPSC, and whatever other shoots i can find. Of course, Much plinking will be done. I just want a fun, accurate shooter. I'd like to be able to do some fast follow up shots, especially for the IDPA shoots.

What is more accurate? the XD9 or XD40?

Any difference in ammo price?
difference in recoil?

One thing i like is the higher capacity of the 9mm hanksPaul

The 9 is going to be cheaper to feed, the cheapest prices for ammo I've found are online (even with shipping).  9mm for about $90 per 1000 and 40 for about $120 per thousand plus about $30 shipping works out to be a little cheaper than WWB at Wally World.

I use a 40 Tact in IPSC/USPSA Production class with some soft loads from Miwall (don't have time to reload).  I run it with a 16lb recoil spring and she shoots as flat as anything with nice quick follow-up.  That said full power 40 loads kick noticably more that 9 but not that much IMO.  As far as accuracy goes they're pretty close in my opinion, can't go wrong with either one.  

I'm not all that familiar with IDPA but as far as USPSA/IPSC goes you'd pretty much by limited to the Production division with the 9 (not a bad thing).  With the 40 you could shoot in both Production and Limited-10 (you could should a 9 in Limited-10 but a 40 lets you take advantage of major scoring).  

The one thing I would do, especially if this gun will be just for gaming, is get some trigger work done.  The stock trigger isn't horrible but there is a good bit of overtravel/reset.   I sent mine to Rich D at http://www.canyon-creek.com and it is fantasitic; nice and crisp with very little travel/reset.  I had mine set to 3lbs but he can do them as low 2-2.5 for competition and does carry triggers around 4lbs.  I had him do the trigger, sights, and a refinish since he milled the slide to take Bo-Mar sights; great guy excellent work, my blaster was only gone 6 days door to door (factory lead time is 2-3 weeks usually).

40

Then there is the up and coming 45ACP Tactical...

40 and then a 9mm conversion barrel.  Best of both worlds.  2 weapons in one.

I've never shot the 9mm, but I really love my tactical .40. I highly recommend it.

IF YOUR CONCERNED WITH RECOIL, CONSIDER A V-10.  I HAVE IT IN .40 AND LOVE IT.  RECOIL IS SO MINIMAL AND TARGET RE-AQUISITION IS VERY QUICK.  ACCURACY IS NOT AN ISSUE WITH THE XDS, BUT WITH THE SHOOTER....GET THE .40

my 9mm tatical is a sweet shooter it has very low recoil

I have both the 4" XD9 and the XD40 Sub compact and the 40 SC is as accurate as the 9mm.  The XD9 is going to be much cheaper for ammo.  You can't go wrong with either the XD9 or the XD40.  I'm getting ready to purchase the XD .45ACP as soon as they come out in SS.

for shoots I would go with the 9

cheap ammo, lower recoil and high cap mags.

for personal use I think anyone would go with the 40

tag

I absolutely love my XD-40 Tactical in OD.

I wouldn't trade it for anything, but the XD-45 ACP Tactical will be getting a good hard look come time to buy another pistol

WIZZO

XD9 Tac will be cheaper to shoot and has a little less recoil.  Look at IDPA and IPSC/USPSA rules for major/ minor factors in the class you would like to shoot in.  Personally I have a XD .40 tac and love it.

Quoted: 40 and then a 9mm conversion barrel.  Best of both worlds.  2 weapons in one.

Not going to work.

Different Breach Faces

Get an XD-40 V-10.  The porting makes a huge difference.