It's not a bad question. I really liked Kafel and I don't like Nadal too much, but trying to be objective, throughout his whole career Kafelnikov achieved more. Of course we can discuss if his times were more difficult or Nadal's times are (and I think at the Kafelnikov's era it was more difficult to grab the important spots).
Also mind that the Russian won the slams in doubles, which isn't easy at all, and the slams he won were on different surfaces.
Kafelnikov is a better poker player, Nadal is probably a better playstation player
Next year my answer to this question may be different. Or may be not, if guys like Djokovic and Tsonga keep developing.