RE: Pellets vs. Powder
It is really all about Trial and Error with developing your most accurate load. If your gun likes 2 x 50 gr pellets than stick with them. If not, then I would suggest buying some loose T7 and trying to tweak your load to see if you can improve upon the pellet load. I wouldn't bother trying the pyrodex pellets as it is the same charge level (100gr of T7 is the same as 100gr of Pyrodex pellets), just different characteristics (ie: smell, more smoke and more elbow grease when cleaning).
Pellets
Advanatage: ease & same charge
Disadvantage: Cost and limited customization for load developing
Powder
Advanatage: Cheaper and full customization for load developing. In T7 15% more potent than Pyrodex (so 100 gr of T7=115gr Pyrodex)
Disadvantage: requires a bit more effort
Good luck