Tough one, but I have to go with Federer. It's really hard to win tournaments when you are playing a series of head-to-head matches. 1 bad day, or 1 opponent who's game is made to beat you, and you're out. Tiger often survives a bad round to win a tournament.
I'm not criticizing Tiger. Golf requires a mental toughness that is comparable to Chess -- plus you got to hit the shot.
Wow.... great point.