It's difficult to disagree with Alex Massie's conclusion on the topic
None of this is to say that Tiger isn't greater than Roger, merely that these sorts of parlour games, though entertaining, are pretty pointless. why isn't it enough to just acknowledge that Tiger Woods is the greatest golfer of the modern game and Roger Federer is probably the finest tennis player of them all (or, at least the finest since Rod Laver)?
I'll add a few things though. The grand slam record for golf really is much more impressive than the one for tennis. Sampras is the only plausible candidate who really had a long career attempting all the slams. Laver didn't play the tournaments at all for 5 of his best years and Borg retired when he was really young. So I wouldn't consider breaking this record as important in the debate (though it means that Federer must break it sometime).
The nature of the different sports means that Woods can have the greatest year ever that includes many bad days (even after taking the randomness intrinsic to the sport into account) while Federer can't, his baseline loss rate is just too low. Also, when Federer has one of his great performances it's just an aesthetic thing, it doesn't really help him with tournament wins or his win-loss ration (except to keep him off the court, which is a small advantage) but with Woods it does help. How many times has Federer actually needed his best tennis to win (not on clay against Nadal, which has not actually happened yet)?
For some reason tennis stars often face extremely steep drop-offs at the end of their careers, and this often happens pretty young, younger than other sports that rely almost exclusively on athleticism. If we accept that this is true, why should this be and what should we conclude about this?
The match play comparison is interesting, but as Alex says, it only gets you so far. I think the best indication comes from the general level of awe each player inspires in observes and fear/hopelessness in competitors. Woods probably has the edge here though Federer has achieved what he has despite never really laying to rest the feeling that he's psychologically weaker than some other competitors, which should throw his sheer talent into sharper perspective.
Lastly, my impression is that if we could be bothered (and nobody in their right mind should be), reading sports journalism from the 60's and seeing how Laver stacked up to other sporting heroes would reveal that Laver did not quite have the other worldly talent that Woods and Federer have.