Mission dictates gear.
What do you want to do with the gun?
If you want to suppress it, the shorter pistol is going to be easier to work with.
Conversely, if you want to push the envelope and pin and weld a carbine, you can probably go down to 13.7 inches and still have a relatively small and handy suppressed gun that actually has a suppressor setting. Still, depending on the can, anything over 11 or 10.5 inches gets kind of long once you start adding silencers.
If you want a non NFA firearm that will probably shoot ridiculously soft, get the carbine.
If you want to shoot groups, get the carbine. It will more than likely be more accurate due to the heavier profile barrel but the differences will probably be minimal.