I'll review these for you :D
A mini-Arceus. With decent stats all around the board, it can basically run any role it feels like. Like arceus. Baton pass? Got it. Sweep? Got it. Wall? Got it. Like a Smeargle, but not as cheap...
Another little cute Pokemon, Celebi also has great stats, same as Mew! It, however, has many weaknesses, but can still do much damage as a Nasty Plot sweeper.
I can't specify much, as both of their movepools are pretty big. Mew can literally function any role, but Celebi can resist common attacks in OU and strike back.
Overall, I think Mew is better. It's more difficult to predict, and they both have the same stats, although Mew has a bigger movepool. Like I said earlier, Celebi has better typing, but mew wins in the end.