I think both legendaries are really cool, and have neat designs! I like them nearly as much as Ho-oh & Lugia, which is my fave pair of legendaries. (I know Zygarde makes it a trio, but I'm not including it in this case XD) I also thought their involvement with the plot was rather interesting, what with Xerneas bringing forth life and Yveltal destroying crap. It made them sound much cooler and more powerful; if only the battle against them were like that :< Of all the playthroughs I've done, I've managed to catch them after using one-three balls.
Anyway, although I do like them both, I personally prefer Yveltal over Xerneas. I'm very fond of the Fairy type, and I find Xerneas rather useful competitively, but Yveltal comes out superior in terms of design, typing, and backstory. I also like its shiny color palette more it looks like flying bacon; I like both legendaries' signatures moves equally, as Geomancy's pretty cool-looking and useful if utilized correctly while Oblivious Wing sounds deadly (especially if going by its Japanese name, Death Wing) and restores Yveltal's HP based on 75% of the damage dealt.
