I loved Mewtwo in the movies! :) He's just so cool. Mew, on the other hand, is super adorable. <3
In-game, Mew has a much wider movepool than Mewtwo. He can learn every single move there is! Just... wow. ^^
Stat-wise, Mewtwo is better, with 80 more overall points that Mew.
The bottom line is, it really depends. :) If you'd like to focus on a certain stat and enhance that, matching it to moves of the same type as the Pokemon, then choose Mewtwo. Mew is more of an all-around Pokemon who doesn't really excel in a certain stat. But it's excellent in it's own right.