The ammo you choose is probably more critical than caliber choice.
I would, as others suggest, go with at least the .44mag...then you could have the option to download to .44spl to practice. If not the .44, then as Haz suggested, I would go .454 Casull because of the option to shoot .45 Colt. Of course, you could also go with the .460 S&W Mag and shoot .460, .454, AND .45 Colt.
Anyway, regardless of caliber and gun choice, the big issue would be proper ammo for grizzlers and such threats. I'd go with something in the 300+ grain range @ 1200fps with a solid core for penetration purposes. Something like the 44 Rem Mag 300gr COR-BON Hunter FPPN [HT44305FPPN-20]. Guaranteed to rattle your teeth....but then again, if a bear is after you, your teeth ain't the first thing on your mind.