OR/AS has more new stuff introduced, but X/Y has the trainer customization which I like a lot. All the new items and a few new attacks and formes introduced in OR/AS can't be obtained or used in X/Y (you'd think with the 3DS and eShop + updates being possible, they'd be able to make an update for X/Y that adds all that new stuff, but no...)
If a Z version comes out, it should have all the new stuff from OR/AS included, and probably some more of its own. I really hope they include more post-game areas to explore, more post-game in general actually. X/Y were sorely lacking. Heck, even the E4 didn't change at all after you beat the game. Why didn't they do that? I mean, every other game before since like Platinum had an improved Elite Four the second time around (even FR/LG, R/S/E didn't though.)
X/Y had Battle Maison which I thought was okay, for X/Y. I don't like how they just copy/pasted that for OR/AS. They should have included the Battle Frontier as a whole, but they decided Battle Frontier would be too much. Why didn't they at least make the Battle Tower? Even though they're both the same, Battle Maison is just... eugh. Not the same feel as Battle Tower at all and doesn't feel like it fits in OR/AS.