Mega Charizard X loses its Flying Type and gains Dragon Typing, affecting the Same Type Attack Bonus (STAB) of Flying & Dragon moves respectively. It also gets Tough Claws, an ability that increases the strength of physical moves. Its primary boosted stat is Attack
Mega Charizard Y gains the Drought Ability (upping the power of it's fire attacks, and weakening water). Its primary boosted stat is Special Attack. This version of Charizard remains Fire/Flying.
If you have one of each, they should look pretty different in terms of EVs and Movesets. Charizard X would focus more on Physical moves, and drop Flying moves in favour of Dragon ones. Charizard Y would focus on strong Special Attack Fire moves, with Flying for extra coverage.