I'm going with GSP on this one. Obviously, they both have incredible (maybe unmatched) skills, but I think GSP has a HUGE edge in the mental game. I think that'll be the difference.
Favorite Fighters: Frank Mir, GSP, Chuck Liddell, Rich Franklin, Fedor, Eric Paulson (Legend!)
Least Favorite Fighters: The Diaz Brothers, Diego Sanchez, Brock Lesnar, Matt Hughes