French Open is much more difficult to dominate.
Nadal has 11 French Opens, the next man has 6 (Borg), and then it's all the way down to 3 (Kuerten, Wilander, Lendl)
Federer has 8 Wimbledons, the next man has 7 (Sampras), and then it's 5 (Borg), 4 (Djokovic) and 3 (Becker, McEnroe).
Clearly what Rafa has accomplished is much more difficult because clay is just more difficult to dominate as history shows. With Wimbledon, if you have a great serve it's already half the battle, with the French Open you have to fight for every point like it's the last day you're on earth and you have to have a complete game. Tons of players are fast and defend well, but it takes much more than that.
Clay is the most difficult surface and that makes Rafa's clear dominance of it even more spectacular. Frankly what Federer did on grass isn't that much greater than Sampras or even Borg. Also give Djokovic a few more years and he might join them. Nadal is already far away by himself and he'll probably add many more.