If you are really serious about purchasing a back up gun for bear then the Alaskan is the odds on choice. Check out the ballistics on the Alaskan and you will find that, even with the short barrel, the 454 will give you about 40% more power than a "comparable" load in a 44 magnum (even out of the 4" barrel). You can always shoot some of the heavier 454 loads and make an even stronger case for that caliber.
If 44 magnum becomes the choice then I suggest that you opt for the 4" Redhawk but, once again, if back up for bear is the goal then carry a hot load (Hammerhead) by Garrett. Bears are tough customers (I know from experience) and power and quick deployment to the first shot are of the essence. FWIW.
As I own both guns my personal choice has been the Alaskan with heavy, heavy 454 loads. No fun at all to shoot but they will do the job.