RE: Triple 777 vs. Pyrodex
Why not purchase one pound of it, spend a day on the range and see how your rifle likes it. I think Triple Se7en is a great powder. There is the chance your rifle might make a hard crud ring, then again it might not. A simple swab is usually all that is necessary to remove the crud ring.
Triple Se7en is much more expensive and it is 15% more powerful like mentioned with the loose powders. So load accordingly. As for it being cleaner, well you have to clean the rifle at the end of the day anyway. If it takes you five minutes more because you shoot Pyrodex, then it takes five minutes more. Pyrodex is more corrosive, so you should always make sure you clean the rifle at the end of the day. Triple Se7en is more forgiving in that respect, but still, I clean my rifle...
If your getting good accuracy with the Pyrodex then remember that and keep a pound of that around. You did not mention what kind of rifle your shooting...