I used to carry a 9mm...until the idiot who shot up Ft Hood took four rounds and is still around. Upgraded to a .40 S&W and haven't looked back.
I've said this before, but the best handgun round out there, or so I've been told, is the .357 Magnum. I know, I know, shot placement is the key. But IMHO I think people harp on that a little too much. If placed just right, a .22 will kill a BG just as dead as a .357 Mag will...IF it's placed just right. So why aren't we all carrying .22s if "shot placement is the key?"
The reality is that most folks in a life and death situation are unlikely to be able to place their shots with the same level of accuracy they demonstrate during practice, simply because of adrenalin and fear. Therefore, a round sufficiently large and fast enough to get the BG to stop what he's doing if he's hit just about anywhere REALLY is THE key.
Just my $0.02.