Assuming the size of the gun isn't an issue, 9mm carbine. It is fun to never miss, right?
Long guns are just inherently more stable. Longer sight radius helps. (or a scope of some kind) The round, out of a pistol is good to 50 yards. Out of a carbine, probably 100 yards.
Not 100% sure of the velocity gains, but I'll bet it is at least 200 fps faster out of the carbine.
45 ACP would also be fun, but it is a pretty slow round, so there's not as much advantage to it out of a carbine as a 9mm, IMO. The ammo's also cheap enough to shoot some quantity, even if you don't hand-load your ammo.
What guns are you thinking of, carbine-wise?
Ruger made some a few years back. Marlin too. Beretta still makes theirs. Kel-Tec seems to be the most popular one now. (Sub2000) I'd like one of those, but I'm not going to get raked over the coals for it either.
I just bought myself a nice 44 Magnum carbine, a Marlin 1894 lever action. Should be a really fun shooter.