Let's compare the two, shall we?! :D
Moves
Mega Charizard X:
Fire Beard @ Charizardite X
Ability: Tough Claws
Nature: Adamant
EV's: 144 HP / 252 Atk / 112 Spe
- Dragon Rush
- Earthquake
- Flare Blitz
- Roost
Dragon Rush is the main STAB and deals decent damage without locking you into a multi-turn attack with the chance of being easily killed by a Fairy-type (cough cough Outrage cough cough). Flare Blitz is an amazing attack (imo), although the recoil can quickly stack up, so Roost is there to help make Charizard last longer. Earthquake is for coverage (and who doesn't love 100 power with 100 accuracy?).
Mega Gyarados:
Giant Shrimp @ Gyaradosite
Ability: Mold Breaker
Nature: Jolly / Adamant (want more Speed or Attack?)
EV's: 252 Atk / 252 Spe / 4 Def
- Waterfall
- Earthquake
- Ice Fang
- Stone Edge / Dragon Dance
Waterfall leads the set, being also the primary STAB with mediocre flinch-chance. Ice Fang is for the obnoxious Grass and Dragon-types (Breloom and Garchomp, to name a few). Again, Earthquake provides good coverage. If you want even more coverage, go with Stone Edge. But a quick Dragon Dance can turn Mega Gyarados pretty deadly. However, there is no recovery, but Gyarados is bulky enough to endure some hits.
Stats
Mega Charizard X:
- HP : 78
- Atk: 130
- Def: 111
- SpA: 130
- Def: 85
- Spe: 100
- Total: 634
Mega Gyarados:
- HP : 95
- Atk: 155
- Def: 109
- SpA: 70
- Def: 130
- Spe: 81
- Total: 640
While both Megas are destined for Sweeping, Mega Gyarados comes out slightly superior in attack, beating Mega Charizard X by 25. Mega Gyarados also has higher special defense and HP, making it a tad more bulky than Charizard. However, Mega Charizard X is better in terms of defense (but only by a measly amount of 2), speed, and special attack (which means you could also run a Heat Wave/Dragon Pulse set).
Synergy & Conclusion
Both Mega's are vulnerable to Fairy-types, so be sure to have backup such as Aegislash, Gengar, and Dragalge to murder any threats. Bullet Punch Scizor is also an option, although Mega Scizor wouldn't be viable.
Being Burnt is a major problem that frequently happen to most Physical Sweepers. Charizard X is better in such a situation because it has burn-immunity.
In the end, it's best for you to experiment with different options yourself and come up with the best member for your team, since the two are very similar.