Depends do you want a tank or a sweeper trevenant has a high 110 base attack and gourgeist has an amazing 120 base defense all of their other stats are average or just horrible. trevenant can kill fast with physical moves and due to it's cursed forest ability you should add a poison move (physical) and you get a pretty good set up going the best trenevant move set is cursed forest, horn leech or wood hammer (STAB), shadow claw or phantom force (STAB) , poison jab (with cursed forest) . gourgeist can last very long in battle and it's best to use defensive moves it's trick or treat ability helps you kill with a ghost move like phantom force or astonish. it's best moveset is trick or treat, phantom force or astonish (STAB) astonish if you wanna take flinch chances. seed bomb (STAB), leech seed (This is what's gonna keep you alive during battle)
hope I helped!