Federer.
For me, golf is tear-inducing boring, while tennis is much more my thing.
Tiger's (1) career slam and (2) overall longer career are about the only thing greater in my mind that Federer's lack of a French Open. Otherwise, everything else lines up Federer.
The argument that it's Tiger vs. 100 others and Federer vs. only 6 others is about the greatest poppycock I've ever heard. Next thing you know, when they institute a playoff system in collge football, the #7 seed will complain that they got cheated because they never played the eventual winner in the tournament.
If Tiger wasn't American, this wouldn't even be an argument.