GSP is a better striker than Diaz, but he's also at his best when he's pressing forward and pumping that jab, then jumps out of range. He isn't a powerful leg kicker, nor have I ever seen him fight like Condit did against Nick even against good strikers like Alves and Penn.
Unlike everyone else, Diaz won't go backwards, but keeps coming forward and is also one of the few guys who never fears a takedown. Nick comes forward and throws punches, GSP shoots. Nick wins the stand-up if GSP decides to stand.