Both are fine players, and Federer can already be classed with the all time greats. I don't follow it enough anymore to have a strong preference either way. (When I played competitively in my teens, I used to watch every match religiously). But I did read that Federer is playing to join Sampras and Borg as the only men to win 4 straight Wimbledon titles since WWI. If Nadal wins, he will be the first to win the French Open and Wimbledon in the same year since Borg in 1980.
Federer seems to be the next great one. I haven't really followed tennis in a few years. I've always liked the great American players, McEnroe, Connors, Agassi, Sampras. The closest thing we have right now is Andy Roddick but he's no where near the level of Federer.