While X has a better typing and a great ability, I think Y wins here. It can hit very hard with Fire Blast and SolarBeam thanks to Drought. He has a ridiculous 159 base SpA and Drought. Fuck rocks, this thing is going to be a major threat in OU. That said, it might be interesting if X made it into OU as well, to keep the opponent guessing when they see Charizard in team preview..
Comparing ZardX and ZardY is, for the most part, irrelevant. ZardY is a wallbreaker that punches holes in teams while ZardX is a set-up sweeper also capable of going mixed.
Comparing them based on what they bring to the meta though, hm. ZardY, coverage-wise, is like Volcarona. The thing they differ in is that ZardY offers more immediate power (Drought + higher Sp.Atk) where Volcarona has to find that free turn to set-up in order to sweep.
While ZardX is added to the list of dragon dancers dragonite, salamence and haxorus, each having their own niche but all being potent enough for OU.