Nope, definitely the Y version, and here's why.
With Mewtwo and now Charizard, I've noticed a common occurrence. In both cases, the X version gets a type change, while the Y version is a strict power up. This is likely also related to the theme of the games, which since early on has been speculated to be genetics / nature / artificial. The X stones in the game will artificially alter pokemon, forcing them to go through a mega evolution into a whole new pokemon. Y stones however, will bring out the natural power of a pokemon, mega evolving them into what would naturally be their peak strength and abilities. Colress in B/W2 had different theories on how to bring out the power of a pokemon, and whether it could happen naturally or artificially, and this is likely the natural progression of that thinking.
Regarding Xerneas / Yveltal, they likely represent natural power and artificial power respectively, and are awakened / brought out by a high concentration of the opposite.
So any mega with a type change we've seen is likely from an X stone, and any mega without a type change is from a Y stone.
