The Fiend subclass gets Dark One's Blessing, which is very useful, as well as the chance to pick scorching ray. Basically, you get health whenever you reduce enemies to 0 health and at level 3 or 4 you get to pick a spell that does 3 attacks and thus triggers your hex curse three times. And it does 2d6 damage per attack, so potentially 9d6 damage, which is pretty decent.
The GOO subclass gets to pick from a spell list that has two additional spells at level 1, neither of which are worth writing home about, and that's it. In time they'll probably get more stuff, like telepathic abilities that might play a role in dialogue, and they'll have more status-affecting spell options, but right now they're dramatically inferior and borderline pointless, save for the RP aspect of being a servant of a probably more or less neutral'ish eldritch abomination rather than a literal denizen of hell.
