I have pokemon Y. It's a fun game. Here's some version exclusive just in case:
Pokemons in X Only:
Staryu/Starmie
Pinsir
Houndour/Houndoom
Poochyena/Mightyena
Aron/Lairon/Aggron
Lileep/Cradily
Anorith/Armaldo
Sawk
Swirlix/Slurpuff
Clauncer/Clawitzer
Xerneas
Pokemons in Y Only:
Shellder/Cloyster
Omanyte/Omastar
Kabuto/Kabutops
Heracross
Larvitar/Pupitar/Tyranitar
Electrike/Manectric
Purrloin/Liepard
Throh
Spritzee/Aromatisse
Skrelp/Dragalge
Yveltal
Mega evolution is X Only:
Charizardite X
Tyranitarite
Manectite
Pinsirite
Mewtwonite X
Megas evolution in Y only:
Charizardite Y
Agronite
Houndoomnite
Heracronite
Mewtwonite Y
I'm not 100% sure on the exclusive mega evolution, so i may be wrong. I would recommend checking out the exclusive pokemons and choosing which one is your favorite
Get both! My brother has X and I have Y. I think Y is fun. I would personally keep Y. Now, if you wanted to battle someone with a Xerneas and you have a Yveltal, you would lose HORRIBLY. I like Pokemon Y but Pokemon X is also a great game.