I'm sure this has been asked before but I can't fing it anywhere.
Does a mid-length offer performance advantages over a carbine?  
Given the same 16" rifle, what will the mid-length do better or more effeciently?

A midlength will have less gas pressure coming down the gas tube, which may lead to a longer lifespan on your moving parts and gas tube.

I also think that the middies look better than the equivalent 16" carbine.

not in my book

Middies' lower gas pressure equates to smoother operation, which leads to several things.  The first is less abrupt cycling, which by itself can help reduce time to acquire a target for follow up shots.  It also means less "banging around" of the innards, so there's less stress on the buffer tube threads, hammer pin and hammer pin holes, and so on.

As for aesthetics, that's just a matter of taste.  My second AR was a middie, and I think it is a nice look.  My third?  Carbine.

The advantages of the midlength gas system over carbine are that from both human and mechanical engineering the midlength is a more optimal configuration for a 16 inch barrel length.  With a direct gas system like the AR has, matching the gas system length to the overall barrel length is critical to optimum operation of the firearm.

Human:  Longer sight radius, more handguard length, smoother operation.

Mechanical:  relationship of pressure to dwell time much closer to optimum than carbine length gas system, so less wear, higher reliability, reduced stress on components, reduced gas port erosion.

Here's a brief selection of duplicate-ish threads.

http://www.ar15.com/forums/topic.html?b=3&f=118&t=405205

http://www.ar15.com/forums/topic.html?b=3&f=118&t=399638

http://www.ar15.com/forums/topic.html?b=3&f=12&t=384703

http://www.ar15.com/forums/topic.html?b=3&f=118&t=395547

http://www.ar15.com/forums/topic.html?b=3&f=4&t=398605

http://www.ar15.com/forums/topic.html?b=3&f=118&t=395207

http://www.ar15.com/forums/topic.html?b=3&f=4&t=391952

http://www.ar15.com/forums/topic.html?b=3&f=118&t=393061

I've never fired a carbine in auto, and it was years ago when I had a couple of shots in semi.  My brothers Rem R-15 appears to be a middy with an 18" barrel, that seems as, if NOT smoother, cycling as the 20" rifle system.  It might be he was using .223 instead of 5.56.

There is evidence the Military Carbine M4 is suffering more stoppages and malfunctions than the Rifle version Military M16, that's arguably apples and oranges, because an AR version of each won't do full auto burst, which is much more stressful.

I also saw lots of posts from people that have fired both, and say the cycling of the middie is far smoother than the carbine.  SO, for my build I decided to go with a Middie.  You'll see lots of posts from people with Carbines that will tell you they never have a problem.  And I would think there is nothing that should stop you from having a Carbine if that is what you want.  Just from a pure pro-con list, all things being equal, I see a lot of poeple weighing in on the middie to be smoother and less likely to suffer some problems a Carbine could develop, BUT no one has presented real evidence of that.  NOTE: I said all things being equal, I'm sure a poor quality Middie would suffer problems before a high quality Carbine.  As well, it wouldn't surprise me if reliability and wear would be splitting hairs with No real evidence either way, the choice may be more the factors listed above. i.e. smoother lesser recoil, longer sight base, etc.

Don't forget the market right now, its difficult to find anything, and when you do have an opportunity to get something at a good price, you're probably smarter NOT to pass it up.  So, I think you'd find a lot of people would agree in this market a Carbine in hand is better than a Middie on Back-Order.  Unless you see advantage you really want, if you grabbed a more common carbine if you found it, likely you would be a happy carbine owner like lots on this board.

What about a 14.5" barrel?  You see some nice ones, like the Noveske Afghan, with a middy gas system.  Same advantages as the 16"?

Might take a look at this thread for some discussion of 14.5 midlengths.  IMO the midlength gas system is more appropriate for a 14.5" barrel than carbine, and midlength is actually a little short from optimum on a 16".

16" is the min barrel length, for less you need to get a bunch of paper work done and pay a tax stamp.  You can Permanently attach a flash hider/comp that is long enough to bring the length up to 16" and avoid that paper work and fees.

Sorry, I have no idea, how well a 14.5 middie works, I did my research for 16" middie.   And haven't seen any real evidence that a middie would be more reliable than a Carbine, just a lot of opinions that appear to be reasonable to consider, along with real factors like the longer sight base, etc.  Hopefully, someone that has experience with both 14.5" and 16" middie weapons can give you an opinion.

Gamma762, that's a good read, thank you.  I could go over this stuff all day.  Any data or testing done on any of this?