CS% is hard to compare because of who running is, etc. I don't usually look at that comparing defensively.
I also want to see how a catcher manages a pitching staff, and from all accounts, Mac is one of the best in the league.
McCann's ISO power is 50 points better than Mauer's. He's a better slugger and RBI man. Mauer's 2009 was an abberation.
Plus, McCann has been more durable than Joe. You hear rumors all the time of them moving him, while Atlanta has no intentions of moving for a while.
Now, I'm not trying to discount Joe as a player. He's a HOF type player. But I think people get waaaaayyyyyy to caught up with him. I believe he's severely overrated, not because he isn't that good, but because the media thinks he's all that. You think he's the third best player in baseball? Please.