This is the first generation that I absolutely love both mascot legendaries(1st gen and remakes don't count since mascots weren't legendaries).
I like Xerneas, but Yveltal is so much cooler. Lugia has always been my favorite legendary, and I have always thought "They should make an evil looking red and black themed version of Lugia, I would love that." Well, Yveltal is pretty much that, and I love it.
But I will definitely be getting Y version.
1. Yveltal, as mentioned above
2. Better cover art
3. Red and Black are my favorite colors, and Y uses those two.
But that's about it for now. I wonder what all will be version exclusive?