Growing up in Michigan (when it was shotgun or archery in my area) I'll say this much. The right 20 ga load will dump a bear as fast or faster than a 30-06 at close range (basing this on the bullet diameter, energy, size of wound channel, etc.) Though a lot depends on your shot placement.
That said, either one would work just fine. I'd stay away from the remington/win/fed rifled sluggers (5/8 or 1/2 oz) and use a rifled barrel with lightfield hybrid sabots (7/8 oz iirc). The lightfield hybrids were extremely accurate, and fairly forgiving of bore diameter differences in different guns when I used them. Hopefully they still make them. Haven't used any since Michigan allowed straight walled rifle cartridges for deer.
The lighter weight rifled slugs would have enough energy at 30 yards, but accuracy wasn't great from my experience and they lose energy and speed very quickly.