Except for possibly DPhillips I wonder how many others who answered here have actually delt with big bears up close, much less had to stop a real charge.
My personal preference for the past twenty five years has been my short 458 Win Mauser bolt rifle but in answer to the original question I would un-hesitantly take a big double rifle over any shotgun. I have used shotguns on black bears but having to face a wounded, charging Brown or Polar Bear with one would scare me to death.