I'd say Mass Effect 2. I'm interested in both, but I'd get Mass Effect 2 first. Bioshock 2 is an odd thing, with the original leaving no real plot threads that would give it reason to have a sequel. On the other hand, Mass Effect 2 not only could have a sequel, it almost demanded it. Not to mention, Mass Effect had, in my opinion, the better story. Also, while both games gave you the opportunity to explore their respective worlds, Mass Effect did a much better job of it. In Mass Effect, there were all sorts of areas you could stumble upon just by screwing around. There were also quests which made to world feel bigger than what I was seeing, and even the smallest things in the game will come back in the sequel, which is pretty amazing. By contrast, Bioshock only allowed you to explore the main story areas, no other parts of rapture, and from what I've seen, the sequel won't change that. Not that there was anything wrong with the original Bioshock. I liked both, but preferred Mass Effect, so I'm more excited for Mass Effect's sequel.
I suppose it comes down to one thing: which one of the original games did you prefer? Pick the sequel to that one.