Well it sorta depends on what you are looking for. Agassi and Nadal's backhands are, in a way, technically dissimilar. Agassi's backhand was similar to a one hander. His left (non-dominant) arm did most of the work. He really only used the right arm to bring the racket around at the last minute. Look for a transcript of his U.S. Open commentary last year. He talks about this.
Nadal, however, has a much more "active" right (non-dominant, because he is a lefty) arm. Venus Williams is a great example of someone who's non-dominant arm is really, really active. She's a righty... it's almost like she's hitting a left-handed forehand.
I suggest you figure out which style you find more comfortable and go from there. As to the question of "who has a better backhand" I'd have to go with Agassi. The combination of his incredible hand-eye coordination and flat(ish) ball made his backhand a formidable weapon. It penetrates the court more than Nadal's backhand. Further, as another poster mentioned, Agassi's backhand return is better than Nadal's. Also remember that, a few years ago, Nadal's backhand was considered somewhat of a weakness. Finally, you could make the argument -- based on the amount of spin on their respective backhands -- that Nadal has a better clay-court backhand while Agassi has a better every-other-surface backhand.