Regardless of timing, this is what I think:
Nadal has discovered his talents at a slightly younger age than Federer.
Federer plays a type of match that isn't as hard on his body. His career could last another 3-4 years at the top of the game.
Nadal had such early success and has such a hard-core and give-all game that his body is already falling apart.... something that was predicted 2 or 3 years ago.
I think they are both vital to the game. But I honestly think that in two years time Nadal's career could be over due to injuries, and Federer could still be playing strong, but just less often due to other important life factors.
Nadal needs to WIN 10 more grand slams to surpass Federer's total... does Nadal's body even have 10 more grand slams in it... regardless of winning them all?