Studies of officer involved shootings have shown that the biggest factor effecting the outcome of the shooting was control/shot placement. That would push me toward the smaller caliber. A solid hit center mass or head with a 9mm is 'more powerful' than winging someone with a .45 ormissing altogether. [8D]

I carry a 2" .357 and I'm pretty accurate with 4-5 shots but after that many rapid shots your hand is about toast.