Mewtwo is definitely better than Mew.
Mewtwo: Mewtwo has a decent and good enough movepool to be able to sweep madly. It has godly stats and is who I consider to be the best special sweeper in the metagame. He has access to almost every type of sweeping move, and what's more, has Psystrike to make him/her a potential mixed sweeper. Awesome.
Mew: Can learn many, many, moves. Although it can't utilize them as well as Mewtwo. It does well in UU, but you gotta learn, Mewtwo rocks the house in Uber. What's cool about Mew, however is that you don't know what it's gonna do. It could wall, it could sweep, it could tank... But Mewtwo is always all--out sweeper (except those Smogon weirdos, they use "Stalltwo."
So, you can choose, but I strongly recommend Mewtwo. There's nothing to be proud of a good, well, even amazing movepool if you don't have the stats to utilize them very well. Mewtwo has a large movepool as well, even if not as large as Mews, and can do much better. In my opinion.