The argument for the 9 is shootability, using a load that has the ability to punch into the skull. The defensive use of a handgun bear is a finesse game -- warning shot, body shot to discourage, then finally, hit the upper CNS to stop a determined bear. Finesse because you don't have the power to be decisive, like with a long gun.
Once I go to a long gun, I want the ability to skip finesse and bring more power. Ideally a semi auto shotgun with Brenneke slugs, and on the smallest size choice, .300 BLK with Barnes is my minimum.