mass effect and bioshock prbly take about the same amount of time. But mass effect has a sequel coming out, and if you've beaten it, the sequel will use the save data. So that's another reason its a good buy.
Mass Effect for me, it took me a little while to get into it but once you get your head around the story and the gameplay it's a fun game. A lot of people compare it to KOTOR which is fair enough since it's from the same studio but the game mechanics are a bit different. Game for game KOTOR is the better game, firstly when it was new on the PC it looked great and didn't require expensive components to run smoothly and look nice. Secondly the KOTOR was a longer game, when I finished Mass Effect the other week I couldn't help but think the game was a touch short and padded out a bit with needless and repetitive side quests that really weren't that interesting.
As for Bioshock................a shockingly bad game. The gameplay is awful and no matter how long I played the game for I just couldn't get used to it. My biggest gripe is the clunky way you fire you weapons, it just feel or look like your firing your weapons properly compared to a normal FPS game. On the plus side I loved the art direction of the game, it kind of reminded of Fritz Lang's Metropolis (I doubt you would have seen Metropolis since it's a film made over 70 years ago. Rather then site through it, search on Youtube for 'Queen - Radio Ga Ga', their video was based around Metropolis and that will give you an idea of what the game looks like).
I know what Metropolis is, though I've never seen it. Yeah, KOTOR is one of my all time favorite games.
I gave the bioshock and ceville demos a go. Bioshock was all right but it didn't scream "GET ME NOW" and ceville... I think I'll pass. It looks like it has potential but the user interface and lack of vsync was driving me nuts.
Most people recommend Mass Effect over bioshock so I'll probably grab it. It's not a huge universe like fallout 3 though, right? Fallout 3 is fun but I'm too exhausted to deal with a game of that magnitude right now. If it's like KOTOR, that's fine.
The only problem I had with Mass Effect is I got so sucked into the story, I did virtually no side-quests on my first run-through. The biggest plus: immediately after I finished I started a new character and played it through again. Spent ~21 hours on the first playthrough and ~34 hours on the second.
Also, it's got huge replayability. Different character classes, and you can unlock abilities that'll cross from one to another.
the nice part about Bioshock is the story gets better and better as you play, the demo did not do a good job of showing that. I grabbed Bioshock (sold my disc last year) off Direct 2 Drive for $10. Mass Effect is a great game also so keep checking both sites for deals.