Nadal's forehand IS better. Don't be a fanboy. Nadal's forehand is better than Roddicks, and I have no problem saying Fed has a better forehand than Rafa.
Nadal has 10x better movement that allows him to use his forehand. Roddick in fact hasn't used this forehand in ages; when he does, he is usually contesting a slam or winning a title (see Wimbledon 2004, USO Series 2003, Dubai 2008 ).
Nadal wins based on his movement, his better backhand, his better fitness, and his better defensive skills. That doesn't necessarily mean he has an overall better forehand; on HCs, I'd say there are alot of guys with better forehands than Nadal, such as Verdasco, Del Potro, Djokovic, and quite a few others.
The justification of using the number of slams to support the fact that a player has a better shot is preposterous. Are you really going to try to make the argument that Federer has a better backhand than the likes of Wawrinka, Nalbandian, Djokovic, Murray, etc.? Are you going to say that Federer has a better serve than Roddick, Karlovic, Isner, Guccione, that Australian guy (Ball I think)? Don't make me laugh.