Ol Mose usually comes up when grizzlies get mentioned. The bear hunter who shot him is reported to have killed him with either a 25-35 or a 30-40 at approximately 15 yds. I personally would prefer a bit more gun and a bit more range.
No matter. The reason for the difference in cartridges and calibers is simple: Alaska is the largest state in the Union, and the wide variety of cover, vegetation types, size and aggression of the bears, as well as the guide's ability to back you up will influence the suggested minimums. I remember my Dad telling me about when he was in the Navy: a shore party in Alaska armed with M1s defended themsleves from a grizzly. They killed the grizzly, but not before he mauled every single one of them. So I would prefer a bit more gun. Really, IMO you should first decide where you want to hunt, then discuss it with the guide, then get really comfortable with the rifle.