There is no such thing as " stopping power" even a .500 S&W can not guarantee a one shot stop, Heck, there are stories of Asians being hit repeatedly with .50 cal and not stopping, they may have been knocked down, but they kept coming.
Keep the higher round count and shoot till they stop making you need to shoot them (whether it's by falling or running).
With a 33 round spare mag arms and legs will be coming off and your trigger finger will get tired before you run out of ammo. Also, if that doesn't get the job done then you should have brought help or got more practice.