At the release of Pok"mon X and Y, Nintendo unleashed Mega Pok"mon, including two mega charizards for X and Y. Mega Charizard X is way cooler, cuz he looks cooler and nothing like normal Charizard and Mega charizard Y looks like normal Charziard with an extra horn.
Mega Charizard X is stronger, while being able to learn all dragon type moves, whereas Y cannot learn ALL dragon type moves because his Mega Form types remain normal flying/fire, and X's Mega Form types change to Dragon/Fire, and that makes him stronger against dragon/rock/ground/flying/grass/dark, etc.
Yes, Charizard X may be the stronger individual but Pokemon is about team work and I will argue that Charizard Y is a much better pick.
First of he has a better special ability because drought makes him and his whole team stronger and makes all water type Pokemon weaker.
Secondly mega Charizard Y has more resistances since he is resistant to Fairy type, Fighting type, Bug type, Steel type, Grass type and Fire type whereas Charizard X is only resistant to Steel type, Grass type, Fire type, Bug type and Electric type and he is now affected by toxic spikes.