Incomparable, one is a liquid whip, the other is a cyclone. Both are good for different reasons.
Nadal can hit both flat and with superspin when at his best. Olderer can hit flat, but his pure topspin FH is significantly weaker, there is a reason why Nadal has the greatest FH down the line and passing shots. The thing that put Olderer's FH above Nadal a few years ago is the pure flat drive, which Nadal somewhat lacked. After 2008, IMO, Nadal's FH has become the greatest shot in this sport.