Mewtwo is in Smash. Mewtwo is the memorably overpowered toy in RBY/FRLG and early competitive Pokemon. Mewtwo is actually available to players who didn't visit limited Nintendo events or used a GameShark. I gather Mewtwo plays a much bigger role in the Pokemon anime/movies though I'm not too familiar with them myself.
I don't think there'd be much of a comparison.
On the other hand, only Mew is available in Pokemon Snap, so this is a wash.
I would vote Mew over Mewtwo because of Pokemon Snap. I am the one. ---