Probably Vader initially but dooka is way more useful overall especially with Jedi and almost everyone runs at least one Jedi. Dooku is viable at 4* in level 70 matches. So don't worry about him not being at 7*. Imo Vader's usefulness drops at around level 60
Specially since you can't viably farm Vader (yet) - I regret wasting materials and droids on him. The Dookster on the other hand is very well farmable specially when you get to Cantina with lvl 69. Even with only the 2 nodes on Hard you can quickly farm him to 4*. And his stun against Jedis is just delicious .
I would say Dooku all the way. Super fast, great stuns, awesome riposte. 2 shards a day on average isn't that bad of a farm... It's definitely better than Vader's.
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That's why you better start farming. I'm playing the long game! I want a 7 star dooku.