Shoot the caliber you are good with and can practice with (i.e. availability). The real world difference between a hot 9mm, 40S&W, & even 45ACP is nominal. Shoot someone in the head, heart, or throat with either of the above calibers will stop 'em.
I personally prefer higher capacity because odds are that I will be shooting crappier than at the range if my life was at stake.
"You haven't lived until you've died." - various