In game or for competitive?
For in game Trevenant and Gourgeist fill similar but different roles, Trevenant can WoW + Leech seed, Gourgeist can do that too, but Trevenant has better abilities in the form of Harvest and Natural cure. Gourgeist has Frisk, so it can be a useful lead, higher overall defenses (a bit lower spdef though) and can shadow sneak, whoch has saved my butt countless times. Use whichever one you feel goes with the rest of your team better.
For in game just use the one you like the most.
