I will also say GS. He's a monster when it comes to dealing damage and if his Swarm activates Rey, it pretty much means the target is pushing up daisies (get it? Daisies? Because, you know, Rey is played by DAISY Ridley?). Also, from time to time, Swarm summons TWO allies, not just one, which also means instant death in 9 cases out of 10.
Dooku, on the other hand, actually only shines in battles against Jedi. And while there are quite a few of Jedi-oriented teams out there, they are not the majority, seriously cutting into Dooku's usefulness.
So go GS first, get him to 6-7*, then, and only then, get Dooku.