Neither. Mega Mawile and Mega Medicham are good wallbreakers due to their high attack stats and Huge Power, but due to their low speed tier, they aren't that good at sweeping late-game. Good late-game cleaners include Choice Scarf Kartana, Choice Scarf Blacephalon, Volcarona, and Ash-Greninja. Kartana and Blacephalon have Beast Boost, which raises your most powerful stat after each kill, which makes sweeping much easier. Equpping a Choice Scarf allows them to outspeed opposing Pokémon more often. Volcarona has access to Quiver Dance, and Ash-Greninja has Battle Bond. Use a fast and powerful Pokémon that is capable of cleaning through weakened teams. Abilities like Moxie, Speed Boost, and Beast Boost are great for late-game cleaners. Setup moves such as Swords Dance, Dragon Dance, and Quiver Dance are also good.