To chime back in again since I suspect I will be the only outward voice of Mass Effect (I'm not even sure if anyone else in OTB played it except for xir and/or cal), I will give you this question:
What are you looking for more?
An RPG with a deep storyline and many levels of replayability?
or an Action/Adventure game (as I understand Bioshock is) with, arguably (again as I understand and have interpreted from Hezz), a deep storyline but only limited replay value.
This sums things up fairly well. I haven't played Mass Effect (though i will be getting it for PC) but I have played KotOR, which is what you're comparing it with, and that was a great game in itself. Except there's slightly more to the decision than that.
Genre.
Like I said, you can't really compare the two games since Bioshock is a first person shooter akin to an easier System Shock 2, while Mass Effect is a role-playing game. So I'll add that to the factors.
Arguably both games have a deep storyline. Both games are also very fun at least the first time through (or so I've heard in Mass Effect's case, and I would probably admit it would more than likely be true). Both games also contain at least a small amount of replayability, Mass Effect more so than Bioshock, which is only natural when you compare an RPG with an FPS (Mass Effect has different characters, weapons, good and evil endings, and a host of sidequests, possibly more than that but I couldn't say, I'll let Oct confirm or deny anything else, while Bioshock has different plasmids and weapons you could use and good and evil endings).
Which leaves us with Genre. What do you like more? An RPG or an FPS? I think that may be the deciding factor based on the points that have been made for both games thus far.
My closing statement: I agree with Seraed, why not both? That's what I'm going to do one Mass Effect is released on PC here in Australia!