-If you play under Smogon rules, you have to keep in mind that Mega Lucario and Mega Mawile are both banned from standard/OU. This means that you will have to play Uber tier where pretty much everything is allowed including Legendaries such as Groudon, and Kyogre.
Mega Lucario has adaptability which increase any stab move such as Close combat and bullet punch to x2. Stab moves does way more damage than it usually should. Close Combat deals more damage than a High Jump Kick from Mega Medicham. Mega Lucario has very nice move pool, and its stats are good to make either a physical attacker and special attacker. Mega Lucario is also the fastest Mga between Mega Medicham and Mega Mawile.
Mega Medicham has the ability Pure Power which doubles the attack of the user. In other words, Mega Medicham has a massive attack. You can spam High Jump Kick, and it will does a lot of damage to any Pokemon. However Mega Medicham is not very fast and missing a High Jump Kick will cause you to lose half of its health. Unlike Mega Mega Lucario, Mega Medicham can't run special attacks. The ability Pure Power only works on its attack.
Mega Mawile gets the ability of Huge Power, which is the same as Pure Power from Mega Medicham. Before Mega evolving, Mawile has the ability of Intimidate which descrease the attack of its opponent, which is fantastic because Mega Mawile is a bulky Pokemon. Mega Mawile has a very nice typing. Mega Mawile hits hard but it's very slow; however, it gets a priority move which is SuckerPunch. Mega Mawile also hits very hard with anything you throw at your opponent.
It really comes down to your playstyle or what works best for your team.