Y, I like that X is a dragon, and I think that it's overall the strongest of the 2, but I like Y more because of the drought ability, I like sun teams and also like the doubles format, so it just fits me better. I prefer it's design too because I think it seems more accurate to Charizard.
Would you rather have Lysander use the power of Xerneas to make you live forever like AZ, or use the power of Yvetal to destroy you?