I was thinking about this, and it seems like Y is better, as it is incredibly strong in the special area, and it sets up its Sun. However X has DD, and Tough Claws alongside a base 130 in both attack stat, although a physical set is an obvious choice. So what are their pros and cons? (For SM OU.)
Mega Charizard X
1.Mega Charizard X is only regular weak(2x) to Stealth Rock instead of 4x weak.
2. Mega Charizard X has near unresisted STAB and coverage options.
3. Zard X has more physical defense.
1. As X's attacks are mostly contact based, it is prone to chip damage from Rocky Helmet, Iron Barbs and Rough Skin. This alongside Flare Blitz recoil often knocks it into range of priority or faster attackers.
2. It is a physical attacker and gets annoyed by Landorus-Therian's Intimidate greatly.
3. Mega Charizard X is very prone to spikes and toxic spikes after Mega evolving.
Mega Charizard Y
1. Mega Charizard Y is immune to Spikes and Toxic spikes.
2. It has an excellent matchup and against common VoltTurn offense teams with Landorus-Therian, Magearna and Wash Rotom. These teams struggle greatly to switch into it.
3. Y has more sp.def.
1. Mega Charizard Y is 4x weak to stealth rock compared to X who is only regular weak.
2. Mega Charizard Y cannot break through Toxapex, which is very common on Balance teams.
3. Mega Charizard Y cannot break through Chansey.
Source: https://www.smogon.com/dex/sm/pokemon/charizard/ + experience