Remember their stats. Mega Charizard X has 78 HP, 130 Attack and Sp. Attack, 111 Defense, 85 Sp. Defense and 100 Speed. Mega Charizard Y has same 78 HP, 104 Attack, 78 Defense, 159 Sp. Attack, 115 Sp. Defense
and 100 Speed. Both of them have a total stat of 634. Mega Charizard X has same offense stats, but due to Tough Claws, its Physical Moves will receive a boost. Mega Charizard Y's Drought ability will remove its Water type disadvantage and boost its Fire-Type move by half. Now types. Mega Charizard X becomes Fire/Dragon type so, it will take normal damage from water-type moves, now 2x damage from Rock-type, Dragon-Type and Ground-type moves. It takes half or 1/2 damage from Bug, Steel and Electric type moves, 1/4 damage from Grass and Fire type moves. Mega Charizard Y will take normal damage from Water type moves if Drought is in effect. Also, if the opponent uses Thunder, then it will have decreased accuracy.
So, Mega Charizard X is a Physical Attacker with good Sp. Attacker. Y is a Special Attacker. But your team should have Mega Charizard X because Mega Charizard Y' s 4x Rock type disadvantage and low Defense will make it go down easily.