Nah, Valencia is worth the fee - lets remember, its less than Nani's. And I think initially we would see an improvement on what Nani is offering right now. For the next year or possibly two, Valencia may very well continue to perform the better.
But when Nani comes to his prime, he'll be a much, much better player. He has more about him and what he needs is time and regular involvement in matches. He may not be an early developer but at Manchester United he will one day fulfill his huge potential.