Biggest difference I've noticed is how often doms take initiative - sub Shuu did it every six or so actions I took, dom Shuu about every one or two. I agree that there's little difference - I was really hoping that once they were dom there'd be more or different actions they took, including using whatever toys or costumes I had. On the other hand, making them dominant usually unlocks a few very hot scenes with CGs, so I'll take that. :)