Tigerer vs. Federer

So I've been going back and forth all day with some folks about who is the most dominating athlete between Roger Federer and Tiger Woods. Both are smashing records, both are currently tops in the field. . . and both are in a Nike commercial with Thierry Henry (?) (I know he's a popular "futbol" player but I doubt most Americans get it -- but maybe bravo to Nike for making us care)

I won't tell you which one I think and why just yet, because I'd like to hear from you first.

What does it mean for a player to be dominant? Between Tiger and Federer, who is the most dominant?