don't worry so much about the round. both of the rounds you have listed will stop the bear in its tracks if you hit the vitals.
when choosing I would look more at the gun and hunting conditions then the round(not to say that the round isn't important).
I have a mossy M590A1, but my mini-SASS in 6.8SPC is a bit handier for me while sitting in my stand all ghillied up.