I apologize for not remembering the source of this info. The guy who collected it was on Tom Gresham's radio show back in April. His findings were based on a couple thousand shootings by LEOs and civilians, good guys and bad guys. He reported on some of his stats. [BTW, if anyone remembers who this was, I'd sure like to look him up. I was in the car when I heard it and couldn't write down--or remember--the guy's name or website.] Here's what he reported:
1. Statistics say you're going to miss more times than you hit. They also say that on average 2-3 hits are necessary to stop a bad guy with a handgun, regardless of caliber. That says to me, I may have to shoot six or seven rounds in order to stop one attacker.
2. Larger calibers have a slight edge in that more--but still very, very few--cases of one shot stops were found in the study with big bullets--.45 and larger.
3. Smaller calibers had a disadvantage in that more cases of the party who was shot never being stopped were found with small rounds.
Given those statistics, my inclination is to avoid really small rounds and low capacities. For quite a while, I carried a full size Colt XSE with 8+1 rounds of .45acp, for the same reason some others here have expressed--I can shoot it better than other handguns. However, I have recently gone back to my Springfield Armory XDm-45 4.5" with 13+1 rounds of .45acp. Yes, it has a typical striker gun trigger. Yes, it has a large grip, even with the smallest backstrap. And yes, it's hard to find a good IWB holster for that gun.
But...that XDm deals pretty well with accomplishing what I think I need in a self defense handgun, statistically speaking: good capacity without reload and not too small a bullet. It also weighs 5 oz. less with 14 rds than the Colt with 9.