I look at the corrosion angle this way. When I shot Pyrodex, I would never THINK about letting the gun go 24 hours with residue in the barrel as I knew the patch would come out with rust. With 777, I don't worry about a day or two as I've never seen corrosion with it.
With Pyrodex, I also found that even after cleaning it as well as possible the gun would sometimes corrode if I didn't clean it a second time a week or two later. This never happens with 777, and it does come out with many fewer patches.
If you're very cost-sensitive, the choice is clear though.