I know this goes back up the post chain a bit, but 9mm is fine SD ammo unless you are shooting through a car at your target. I spend every weekend at the range, and I see the guys with the big guns and calibers that couldn't hit a barn from inside it... so their "SD ammo" isn't very good if you can't hit what you're aiming at.
I've got guns in all kinds of calibers from .22LR to .44mag, and each one of them will kill you dead as a rock. It might take more than one hit with some to "stop" you, but most of them will drop you like a stone if you hit the 10-ring (center mass or head). I like 9mm and .40 caliber, because they carry lots of follow up shots and you can carry a lot of ammo by simply putting several clips in your pocket... but I wouldn't hesitate to shoot with my .38, .357, .44, or even my .22's. I'm going to put my shots in your vitals... which is what counts.
Now, If you want to argue "body armor", there are considerations that need to be made, but for general self defense... any caliber will do, and those with +P, or a handgun picture, on the box will work for maximum knockdown in that particular caliber. The .22's have some specials, like "stingers" that are "upgrades", but I don't need to buy Hydra Shoks to knock you down with a 9mm... all I need to do is hit where I'm aiming.
WT