It is a question of gun handling for me. Given the two choices the 300 Mag. with 220's is unquestionably more powerful and likely to penetrate and break bones if, you can bring it into action rapidly and accurately at close quarters.
A short barrel pump shotgun loaded with Brenneke's or the saboted 50 caliber penetrators will also penetrate and break bones. The shotgun would be much faster to bring to bear and with repeat shots. I would leave the buckshot behind, big bears need penetration and breaking of bones....even double ought buck lacks both unless close enough to act like a slug.
So, my choice is the shotgun, with premium penetrating slugs, easier to carry and easier to use at close range.