I mean if we're talking straight up greatness then I would have to say yes, sure. Novak's extra wins/extra finals are nice, but he's a better hard court player, nobody can argue that. He's more comfortable on the surface, and he always has been. Still, to have lost 5 of his 8 finals is pretty, um, not great. This isn't some Federer at Roland Garros situation where all his losses are to the one guy, he's lost the US Open final to 4 different guys. Now I know there are extenuating circumstances to some of these that make them seem less bad than when just looking at the results out of context, but still, we're talking greatness. He's had excuses for losing other finals too, and still won them, so...
Nadal, for sure. There's something that detracts from greatness when you win lots of matches and repeatedly lose the most important one.
Of course he hasn't actually won the 4th one yet, so this is kind of a premature thread.