My vote goes for Bioshock, but only if you're literature-minded like I am. I was able to get so much out of Bioshock's story that I found myself enjoying it more thinking and discussing about it later than really playing. If you're into that sort of thing, go for Bioshock. It's about as deep as the ocean it's set in.
But if it's just the game that you're looking for, I'd say Mass Effect.
But if it's just the game that you're looking for, I'd say Mass Effect.