I prefer Y, he was the reason I picked Pokemon Y over X, I needed that Charizardite Y!
I like that X is part Dragon, he has the better typing I admit, I also like that he is built to set up and sweep with Dragon Dance, and is a little more tanky and Tough Claws is a pretty good ability of its own. However, Y's stat distribution impressed me more with that stratospheric special attack. Its a more natural transition for Charizard to make as others have pointed out, and I think it puts him overall in a better offensive position- Intimidate doesn't affect him, he is not open to damage by making contact and doesn't take recoil damage from a flare blitz.
Likewise I always thought Y's design fits better. I can see why people would like the X design more, cool, very dramatic color scheme and it will also remind some players of Alain, but I think the Y mega looks more realistic as another form of Charizard and I personally always found it a more sleek design. Y also has a good shiny, X is only looks good in normal form.
Really what I love about Y though is that he can help the team more. Drought is such an amazing ability, I love carrying my own sun, speeding up other Pokemon with chlorophyll, countering my foe's weather. He also has great coverage with broken solar beam in sun.
