Nadal and Federer have extremely similar forehands- both straight arm, both very loose in the wrist, racket points to the ground before forward swing, both clear pull strokes.
The differences lie in the grip, with nadal being more extreme he must move through the strike zone faster with more of a brush up motion.
The next big difference is there rackets are as different as could be with Nadal's allowing for a little bigger strike zone to accommodate his grip and swing.
I do think the grip itself is a little over rated as hand size compared to grip size can affect the grip, and even more importantly the swing path and technique really is the primary determining factor, of course hard to hit a double bend wiper with a continental and an over the shoulder on the edge forehand with a western so the extreme grips on either side require a specific technique.
5 Head PT57a 12.3 oz. 11 pts HL. Pair of PT167a for the wife. Nursing a semi, one hand both sides.