Play whatever you'll have fun with.
Having said that, my opinion would be Mystic gives the party a specialist in Mysticism. With some Computers skills a solid science officer. Mystic spells which are a big deal. A few feats into Longarms and a Mystic can still put up decent damage while still being a crowd control beast.
My Envoy will often make it so everyone on the team will have an effective bonus of +3 to +4 to hit specific targets in combat, making even characters who aren't direct damage specialists hit far more often than they normally would.