BigMikey, i am far from any hunting expert, so i wouldn't have the credentials to tell you if a .357 would be an effective calibers in such a rare occurance. like many other's say on this forum, and link to the stories, that pretty much any caliber from a .22LR to .460 have killed bear on many occasions. I think for someone who is to rely upon factory loadings, the .357mag would be the better choice, if talking purely ballistics. But their are some real-world advantages to the colt, the most important IMO being the concussive blast of the high pressure .357 vs the low boom of the 45colt, recoil will also be somewhat less in the 45LC if talking "factory loads". am more interested in the 45 personally because i hand-load, and the action of the lever and the amount of barrel around the bullet allows you to load up to the "Ruger Only" territory, with research and close attention to work-ups. In that regard i thing the old 45 can be the winner if your talking shorter distances of 100yard and under. You can load the 45LC to 44mag levels and possibly beyond if so desired, it's the factory loadings that cannot prejudice the type/age of the firearm being used, so can only safely be sold to what the weakest link can handle. They do sell specialty loading from Buffalo Bore and the like that will reach these levels, but at a high cost, and at that point, why not just go for the 44mag.
the 357 is a very lethal chambering, and it gets even better out of a ten inch barrel like the Mare's Leg type firearm. I don't think the statistical risk and the apples to oranges ballistics between the two, that you could very well say that either in better than the other, and i don't see any black and white reason to not just stay with the much cheaper 357mag and have the benefit of having some fun with the .38spl when at the range.
even factory loads can get the 357mag to bark out a 158gr bullet close to 1600FPS out of the extended barrel of the lever pistol, and the 45Colt can push the heavy 225's at 1300 with premium loadings, faster with the Buffalo Bore. But either way, you can see that they appear to have similar energies, but with different philosophies behind their ballistics. Maybe a more experienced hunter could give you the better answer.
for a non-reloader, i hate to say that the most beneficial of the available chamberings is likely the 44mag. and that also comes with the benefit of the 44spl for fun. Since the likelihood of this firearm actually being used against a bear during your fishing trip are so unremarkable, i would just get the caliber that you want to own when it's also not just there to save your hide. Factory 45LC is stupid expensive and 44mag isn't cheap either. i wouldn't trade my Rossi 92 in 357mag.38spl for anything, if i didn't already own that, i wouldn't be seeking the 45LC, the 357/38 is way too much fun not to have, and even the 92 rifle is about as light and handy as you can get.