Gardevoir, even without the Mega, is a decent Pokemon, and fills much the same role as Sylveon would. It wouldn't have Pixilate, though, so would have to use Moonblast. Don't get me wrong, Moonblast is a good move, it is just that Pixilate Hyper Voice is more powerful, and ignores Substitutes.
I think Sylveon is slightly stronger because of that. Looking at their stats, Sylveon also has slightly better bulk, with more HP and Special Defence than Gardevoir does. But they both do well as bulky Special Attackers, so my advice is to try Gardevoir out and see how it goes. If you don't think it is working as well as you would like, you could keep looking for a Sylveon later.
Also, do you have a copy of Black 2 or White 2 yourself? I ask because I KNOW you can get a Hidden Ability Eevee in the post-game, so you can always get it and breed it in Gen V, and teach it Hyper Voice from the move tutor, before transferring it to X and Y to evolve it. It might take some work, but it is a guaranteed way to get a Pixilate Sylveon with Hyper Voice.