10mm is what people are moving to these days and I think it is a good choice. I know quite a few that carry .357 revolvers and a couple that are fine with a 9mm. Others carry bigger stuff. A .45 ACP with +P loads will work as well. The old school wisdom was a .44 Mag was the minimum. I think loads and bullets for the others have made the .44 less appealing. I think a 10mm is about the level of recoil most can live with and shoot well enough. You have to learn to shoot the hard kickers.
In black bear country I carry nothing most of the time, or sometimes a .357 Mag Colt Trooper. In grizz country, I always carry and it is either a Glock 21 .460 Rowland, or a Dan Wesson .44 Mag.
Two major considerations with any handgun for bear defense. It must be convenient and easy to carry. Or you will not carry it. If you know you should have it, and think, man I hate carrying that thing, get another sidearm, or a different holster. Be warned you'll end up with 14 holsters you don't like and one you do.
It must have suitable ammunition. With the right ammo a 9mm is adequate (not ideal, but it will work in a pinch). With the wrong ammo, it is more of a hazard than a help. The ammo must penetrate from any angle which means solids of hardcast lead, jacketed, or soild copper. Don't skimp on ammo in this case.
Jeremy