I agree that a bigger caliber (,40 or .45) is usually better, but like others have said, it's better to have a smaller caliber than nothing. The point isn't to necessarily kill someone that is attacking you - just put them out of commission. In fact, something like 70 - 80% of people shot with a handgun survive. I work in a hospital too, and most of the cases I've seen come into the ER end up surviving. The ones that die normally were shot in the head, or were shot many, many times. And not even always then- we had a drug dealer that was shot at least 7 times with 9mm hollowpoints by the police - mostly through the chest and once through the neck, and he somehow survived. But in most of these cases, the person shot was not in any shape to continue attacking anyone.

Back to the point, you need to be comfortable with your gun and the ammo you'll be using for self-defense. IMO, any compact, reliable handgun that you personally can handle well, draw fast and shoot with reasonable accuracy out to 7 yards (yes, that's 7 yards - statistics show you are much more likely to need & use your CCW weapon at point blank range than any other range) should serve you well.