•Extremely high Attack stat
•Good Defensive stats
•Reliable recovery move
•Shallow physical movepool.
•Extremely high Special Attack
•High HP and very high Special Defense
I prefer Mewtwo X, but it depends if you need a special or a physical sweeper.
Mega Mewtwo Y is very overrated at the moment, but either way, it might be better because Mega Mewtwo X movepool is abysmal(physical) and his ability is worse, although Y can easilly run a Special set with 154 base Sp.Attack.
However, Mega Mewtwo X is really not that much better than Vanilla Mewtwo (its basically Mewtwo who equip life orb and have higher special defense, the speed does not do jack). Mega Mewtwo X have high defense (VERY important) and does not weak to bug, while Y is harder to revenge kill with Special Scarf such as Palkia.
Hope this helps.