One definitely needs to choose a sidearm that can be used effectively. More gun than you can manage won't be ideal. Alaskan guide Phil Shoemaker used 4 x 147gr Buffalo Bore 9mm hard cast to kill a 1,000 lb grizzly that attacked his clients but I'd say that there was more than a little luck involved. I know someone personally that used a 9mm with 115 ball ammo to defend himself against a black bear. He expended all 30 rounds that he had on his person and the bear...ran off. Literally, he watched it run hundreds of yards away from him, up the side of a ridge and vanish on the other side! The conventional wisdom is that you don't need to kill a black bear to end an attack, just have enough gun to dissuade it. I'm not sure how valid that is and certainly leaving a wounded animal for someone else to deal with is less than ideal.

I don't presently have a big bore handgun but it's on my to-do list.