X.....because I went to the counter to pre order and went "I want to preorder the new Pokemon uuuuuuuuuuuuh X............" Then realised Y is the one with the better legendary.
However with more research I have seen I really do want X, mega evolutions and other pokemon exclusives I want X.
I chose Y because I have a Y in my name, and because I like the color red better than blue. The legendary looks a bit cooler as well, though I expect he will be less useful. Mewtwo Y is also the superior looking version, though it's kinda lame he doesn't get a secondary type too.