As much as I don't want to like the power belts, I keep coming back to them every time I want to try something new. About 20 years ago thier weight retention really sucked. I don't know if they changed anything in bullet structure in all this time, but I have been really impressed with this aspect the last five or so years.
Also, for powder, I've found that Buckhorn is hands down the most consistent.
For whitetails in a 50cal ML, I'd just use whatever shoots best. IMO/E it will be more than good enough. That said, I've had good luck with the powerbelts. One thing I like about them is they are kinda like a sabot, but all in one piece. They don't come apart in the possibles bag/speed loaders making for simpler, faster reloads.
Hornady 45cal 300gr XTP mag bullet inside of the black sabot. T/C packaged these together for years under their name or you can buy the components separately, ex Midway USA use to carry them seasonally in the late summer or fall. The brand name for the sabot was harvester, it was black and designed for a .45 cal bullet in a .50 cal muzzleloader.
I used these on whitetails hunting in Minnesota. To date, I've killed three deer with this combo. One dropped in its tracks and the other two death ran 30-75yds before tipping over.