I've had trouble getting good accuracy with T7 pellets. With T7 loose powder however, I get excellent accuracy. Pyrodex is dirtier than T7 IMO all around. I get somewhat of a crud ring with T7, but it is not too bad. I use T7 primers which produce less of a crud ring than the hotter Federal 209A's. Usually I can load and shoot 5 - 7 shots without swabbing using T7, although accuracy begins to suffer. For me it's not a problem to run a patch down the barrel between shots when on the range.

They are both good powders. If you really want to fine-tune your load, and prolong the life of your powder, I would highly suggest switching to loose powder as opposed to pellets.