I have seen and participated in a HUGE number of bear threads over the last few years. I spend a fair amount of time in both black bear habitats year round, as well as grizzly habitat yearly but much less frequently. My bear count this year (that I've encountered in the wild) is already up to "2" and that's still just through June. So I pay attention.
I will summarize the general concensus I've read over the years as:
10mm/.357 magnum loaded hot and hard: OK if you must.
.41 Magnum: Better choice.
.44 Magnum: An even better choice.
.454 and up: Still an even better choice.
When I am in grizzly habitat, I take at least a .44 magnum. In black territory, .357 suffices for me. With all the vagaries of size and type of bear, and loading of the gun, shot placement, etc., it's just impossible to make a flat rule. I know of AK huting/fishing guides who make a 10mm their sidearm of choice. And have put down grizzlies with that caliber. Others say stick with .44mag.
I will say this as a general rule - anyone who tells you that only a 12 gauge will do has never tried to work/live/ride or recreate in wilderness grizzly habitat. You just can't carry a 12 gauge with you everywhere. You will need to set it down to split wood, carry water, relieve yourself, etc. The gun on your person is the only one you can trust to be there 100% of the time. Even with a rifle in my saddle scabbard, I still keep a large-bore handgun on my waist or shoulder. X-frames and Freedom Arms type guns are just to much for me to lug everyday all the time, so a smaller S&W N frame in .44magnum is my choice.