The Power Belt is a sub bore size, headed lead bullet with copper plating. The SST is a copper jacketed swaged lead core bullet. With a drawn copper jacket, the engineer has several options to use to get accuracy and terminal performance. A lead bullet is just that a lead bullet.
The power belt being sub bore size, depends on a tight bore and a heavy charge to swell the base of it, to engage the rifling. I would bet a steak dinner if you took ten rifles at random used good sights and proper loading and shooting procedure the sabot and the copper jacket would out shoot the power belt 8:2.
Shots at 50 yards, it probably doesn't matter. If you plan on reaching out to touch someone, the sabot has it all over the power belt. The copper jacket bullets have much higher BC's as well, that means less drop and higher energy at extended ranges.
Dry dirty bores or those with heavily lubed bores and cold temperatures are the biggest contributor to hard loading sabots. Sabots will however take more pressure to seat than the power belt, they have too. You are actually cutting plastic while seating it.
Hornady also sells a set of go, no-go gauges to check bore size. Most quality muzzle loaders should have a .500-.502 bore, but with all the import stuff out there, you can find bores of all dimensions. I've seen them as small as .498 as big as .504. I have seen power belts actually fall out of a larger bore barrel when the muzzle is pointed down. But it was easy to load.
Speaking of cold temperatures, those under 20 F, many sabots will get stiff and actually shatter on ignition giving oviously poor performance. The Hornady LNL Speed sabot has been tested to well below that with no failure.