Big, resounding Poison Jab. Here's why:
- Better Accuracy - Always important.
- Good Coverage.
- Counter to Fairy-types, which Mewtwo X is weak against.
- Chance to poison - 30%!!
>252 Atk Mega Mewtwo X Iron Tail vs. 104 HP / 0 Def Xerneas: 306-360 (73 - 85.9%) -- guaranteed 2HKO
>252 Atk Mega Mewtwo X Poison Jab vs. 104 HP / 0 Def Xerneas: 244-288 (58.2 - 68.7%) -- guaranteed 2HKO
So you see, both are guaranteed 2HKO, so the Accuracy of Poison is more useful than the slightly higher power Iron Tail has to offer. (Although, if a Xerneas comes to play, you *must change Mewtwo X, because Xerneas can OHKO Mewtwo with a single Moon Blast).