Depends on the situation.
Most days, I carry a snub .357 revolver because it reliable, powerful, and I shoot it better than most anything close to it's size. I also carry OWB so the weight of this Ruger is not off putting, but a longer barrel catching on things would be.
The downside? It's only 5 shots, and reloading a revolver in a hurry is something I have never been able to master as fast as dropping a magazine and slapping a new one in.
While I know that there is no crystal ball to tell me when/if I will need a gun, I try to play the odds. Walking around the quaint downtown of my little hamlet, the .357 is most likely 5 more shots than I will need in 100 years carrying around there. If I go to the more urban and run down downtown of the next larger city over, the snubbie would probably do the trick, but I do feel a little light on fire power. In those situations, I carry a Glock 26 with two extra mags. I honestly find the extra 5 rounds of quality 9mm in the magazine to be totally sufficient for protection, but with a gun so lightweight as the 26, tucking a double mag pouch onto my belt's weakside isn't all that noticeable, so why not.
In short:
1.) Reliability
2.) Comfort (actually packing the thing)/Firepower (depending on the situation).