Nadal's forehand is more consistent and less prone to errors, but it's also less of a weapon. In 2008-2009, Nadal's backhand was incredible. He hit the most insane flat crosscourt winners off of it no matter how wide he was pulled. But he's always struggled taking it down the line and adding variety.
Federer, on the other hand, has a lot of variety, but his backhand breaks down more. I think they both fit their respective games very well. If push came to shove, I'd rather have Federer's backhand than Nadal's, but it's a close call.