The FBI report concluded that the only way to guarantee that you will knock the attacker on his azz is to hit him in the head or spinal column. Whatever the caliber. Most of the time when bad guys go down after a non lethal hit it is a conditioned response. There were a multitude of documented instances in the report of bad guy who kept coming and shooting after being hit center mass. In fact, there are instances of perps receiving a fatal shoot in the heart who kept fighting back for 20 to 30 seconds before they bled out enough to become incapacitated. Whether you agree with it or not the FBI report is very interesting. There's a two part video on youtube that examines this report in pretty good detail which is well worth checking out. I don't have the link so you will have to search for it on youtube.
Well I own two 9mm’s and its a natural fact that they are easier to shoot quickly and accurately but a lot of the stopping power is ammo. Lucky Gunner has a really good video that you might want to watch comparing the various brands and types of ammo in different calibers.