I am not a GSP fan (not a hater either; sorta neutral), but imho GSP has the superior wrestling (and I think the best MMA wrestler of all time by a pretty decent margin.) Fighters cannot stop GSP from taking them down. It seems like he has very few failed takedown attempts and once he is on top he seems to stay on top. In addition, GSP does not get taken down (ttd). Unfortunately, I do find GSP boring, but that is another topic.
GSP by takedown % and stay on top % and tdd. It may not be a fair comparison though, because GSP outweighs and is able to outmuscle opponents. Cain has to take down bigger guys (both bigger and bigger relative to his own weight.)