Try and find a .45 Colt Blackhawk convertible. That way you get the strong Blackhawk frame, plus an extra cylinder to shoot .45 ACP, it's a heck of a value and a lot of bang for your dollar.
.44 vs .45, it's not really a big deal for black bear. Your .357 Mag with the heavy 180gr hardcasts would work very well on black bear so you really don't NEED either. However, of those two I would have to pick the .45 Colt. Yes handloading will get you the most out of it, but that's with any cartrdige, including the .357 Mag, .44 Mag, etc.
If you want a big bore, might as well get a big bore and the .45 Colt is a bigger bullet for sure (.429" vs .452"). It does have the power advantage over the .44 Magnum, it's just a bigger case that can hold more powder, it's that simple. No doubt some .44's are VERY HOT, but they're loaded to astronomical pressures, likely rivaling 454 Casull pressures. But in the end, you don't need a wrist snapping heavy load to kill a black bear. If you don't handload, warm .45 Colts can be had via Buffalo Bore, Grizzly cartridge, and a few other places. If you don't handload and don't feel like buying your ammo online, maybe sticking with the .44 would be a better bet as it would be more likely to find on the shelf in the store, at least loads loaded warmer anyways as most of the .45 Colt you find on the shelves is about the equivalent to the .45 ACP.
Or, if you can scrape together a few hundred more dollars you can buy a Magnum Research BFR chambered in 454 Casull, or 475 Linebaugh, even the 500 JRH (and 500 S&W) if you want real big bore, but I think the beauty of the .44 Mag and .45 Colt is that they have all the power you or anyone else really needs for 99.999% of all hunting situations.