Go with mid-length. If you're running A2 style sights it gives you a longer sight radius in addition to the already listed benefits outlined by MichiganScott.
The carbine gas system has been proven on 14.5" barrels aka the M4. The 16" barrel combined with the midlength gas system is ideal.
One last benefit, the carbine gas length looks really goofy with a 16" barrel when it comes to proportions. The mid-length looks a lot better.
So that's performance, reliability, and looks all improved. Could you ask for more?
