"Since most pistols chambered for the nine hold more rounds I claim that the added firepower makes the nine more effective overall"
Tell me something. Say you have two guys in a gunfight, on the same team. One guy puts one bullet in each badguy, thus reserving his ammo. The other guy sprays his ammo, and empties three magazines, getting all of three hits (like a lot of big-city police seem to like to do these days).
So. The First Guy gets about one kill for one bullet. His partner gets one kill for one magazine. Tell me who has more firepower?
The point is, having fifteen rounds isn't bad. It's GOOD! But NEEDING fifteen rounds is less than stellar. Don't be lulled into thinking that having more ammo for a pistol, which, as I stress, is a purely defensive weapon, is going to give you some bonus in firepower.
But, back on topic. Any bullet shooting hardball is going to be less effective on a human, simply because the bullet is more likely to go completely through the target, and thus NOT expend all its energy on said target. The 9mm has higher velocity than the .45, meaning it's going to have more of its energy left over after going through a human. Now, since as a rule, 9x19 has less energy at the muzzle than .45, that's quite a bit less energy expended on the target and quite a bit more being used to push the bullet beyond the target into whatever's behind him.
Now, at close range, any bullet of a decent power will overpenetrate using ball type ammunition. The question is how much energy is expended on the target before the bullet leaves his body.
Some of my reasoning may be a little flawed, (it's been a long time since I've had a physics class
), but this is what I figure.