I have been using T7 pellets for a while now, and haven't had any bad experiences with it, accuracy is pretty good. The choice of which propellant to use is yours. I listed what I know about the two brands below:
Pyrodex pellets have a coating of black powder on one end of the pellets to ensure ignition. You have to make sure the black powder side is inserted into the barrel correctly (facing down).
It's been said that use of T7 pellets cause the "dreaded crud ring" (fouling) to form. I make sure I remove any fouling after use, by a through cleaning with soap and boiling hot water. To me it's a non existent problem.
Pyrodex is also said to cause a crud ring to form, but the difference according to people that have an opinion about this is that Pyrodex causes a "softer" fouling than T7.
You also have to remember that since the Pyrodex pellets have black powder on them, they are actually more corrosive. Just make sure you clean your gun after shooting.
IIRC Pyrodex pellets are less expensive than T7 pellets, but that may be just a result of my location.
