Captain Comet would be the more decisive choice in terms of telepathy on the LEGION team, surely.
Since Mantis' actual offensive telepathic abilities have never really been explored (or seen, individually) I'd say Telepath and Comet combined are probably decent enough matches for the dog and Moondragon. Enough where I don't think it will be a deciding factor.
Between Lobo (the only top tier on either side), Lady Quark (herself a near top tier), and Captain Comet I think LEGION has the more powerful line up. Groot will be a decent match up for Strata or Garv, Eagle for the other, Vril will be decent against Starlord, Stealth against Mantis, Lobo is going to dominate Drax, Comet is going to be a weaker telepath but a stronger telekinetic and physically much stronger than Moondragon, oh you know I forgot about Vance Astro...actually, with all those telepaths they might win it regardless, though I still think they are at a power disadvantage.
If they are allowed to bring gear into it, then it's going to be the REBELS no contest. Vril has a 50-world spanning police force of powerful robot enforcers and is a 10th level super intelligence. Figured out a way to stop Waverider, for instance, on the fly, and create an impentrable barrier around a multi-solar system space (so powerful that not even light-speed traveling planet destroyers could get through at it's weakest point).
Weapons are standard equipment for the Guardians. Personally, Lobo seems to be a bit of a trouble, Guardians can always try triple Moondragon/Cosmo/Mantis TK blast -add Major Victory for good messure- which worked on a weakened Thanos. Barring that, they would have a hard time dealing with Lobo alone, if not using bfr or assuming uber pressure points can take him down the count.
There are three things in which the Guardians excell: Martial arts, TP & bombs. If those three things fail they are screwed.
I hate to say what I'm about to say, but I don't see how Guardians can win physically this one. Even if there's Gamora, Mantis and Moondragon on the team, how can they stop Lobo?
Nah, their best bet it's to go all telepathy on L.E.G.I.O.N. Moondragon is a BEAST in telepathy department, having in the past blocked Charles Xavier, mindrape Thor and mindrape an entire planet too. Also, there's Mantis and Cosmo, who are also monsters when it comes to telepathy. If that fail, than L.E.G.I.O.N. wins.