It really depends on your preference. Have you played ANYTHING pokemon before? Like Showdown, Pokemon Online, or any of the MMO's? If not, you should really look into the game a bit more first before you decide. It's basically an RPG in which you use powerful creatures to defeat the enemies, as well as battling random trainers and earning the eight gym badges to get to the elite four. members.
Personally, I would only purchase Pokemon X/Y if I thought I'd be into the competitive or collecting scene, because once the story line is over, that's pretty much all there is left to the game, and the storyline isn't all that lengthy.
I've never played a Zelda game before, and I don't think I'd ever be interested in playing one, but if you love the series that much, then maybe you should stick to what you know, although saying that, I probably wouldn't purchase a remake of a game I've already played before either.
If you want longevity with a never ending game, breeding pokemon, catching them all and completing the pokedex, and battling against friends and people you meet online, then go with Pokemon. But if you just want a game with a decent/good storyline, then Zelda is probably the best choice for you.