With the easy availability of ammunition mentioned, or by hand-loading, there is no significant difference. Either one can kill anything on earth with a hard cast bullet and a good sized meplat. Either one will stop any bear if there is proper shot placement.
The OP specified factory ammunition only hence my previous comments. For a handloader with an adequately strong gun, it's the same situation as with boutique ammo: six in one hand and half a dozen in the other. I would, however, note that even for a handloader the .44 Magnum has a slight edge in a DA revolver since the larger rim allows more positive extraction than does the miniscule rim of the .45 Long Colt. For bear defense, however, speedy reloading probably isn't part of the equation.