Mega salamence isn’t allowed in ou so mega garchomp
Fr though I’m just going to assume you mean like ag for the rest of this answer
Salamence can use dragon dance really well and also has a really great ability (aerliate) which is good with return/double edge. He also could use Earthquake (for coverage) plus his pre mega ability is Intimidate which is also good. I also found out (thanks sumwun) that theres Hidden abilities so I guess you could use moxie but I think intimidate is better
Garchomp can set up stealth rocks and use swords dance but you probably want to pick just 1. He doesn’t have much coverage besides stone edge, doesn’t have the greatest speed stat, and also can’t reliably boost his speed.
i think salamence is better but idk you can decide