Ubers Sylveon (GP 2/2)

[Hack]

This would mean that you cannot use the pokemon in official Nintendo competitions, since the Global Link requires the pokemon to be found in Kalos.

And i find the other ability better, since you can't learn hyper voice in XY naturally.

We don't care about official Nintendo competitions, we care about our simulator metagame. A possibility of getting said moveset with the ability means that it is objectively better in terms of actual benefit. So basically, saying that an ability is better because it is easier to breed and make a mon with that ability doesn't make it objectively better.