I know what happens in George's III. Dooku is dead too quick to really change anything... and then, in his 5 minutes of screentime, they have to reveal him as a Sith yet again to the Jedi, or never make reference to him being a Sith again.
That would be choice b
. The audience already knows from Episode II, and the Jedi already know. We don't HAVE to see them finally decide on it, with that 2 year gap and the audience's recognition of it, it helps narratively to jump ahead a couple steps. Lucas employed the same trick between ANH and ESB by having Vader track down Skywalker for blowing up the Death Star. We saw luke do it, so when ESB starts 2 years later, and Vader knows it too, it's easy to accept that he just figured it out in that space of time. Same thing here. It's pretty obvious he's a sith. And Palpatine uses Dooku to help turn Skywalker. Skywalker's FINAL turn comes later in the film, but Dooku finally realizes too late in the movie what Palpatine is doing--and there's a lot of subtext you're going to lose if you make Dooku NOT a Sith. Palpatine is openly auditioning Anakin and Dooku much like he does Luke and Anakin in ROTJ. You lose THAT subtext as well if you make Dooku not a sith.