hmm I would say get both. (heck I bought 4 cheap PS3 exclusives on Monday for £17.99 each)
I personally liked Bioshock more, but at this price the value of Mass Effect is unbeatable

Mass Effect, it is such an awesome game

You do reallize this is like the hardest choice right?

Go with ME first.

I loved Mass Effect but personally thought Bioshock was the better overall game.

Trying to look at Williams rear in the elevator was a lot of fun though...wait...was I the only one who did that??

Honestly though, I'd be really surprised if you didn't enjoy the hell out of both of these games, so the order you get them in isn't all that important.

Two of my very favorite games. Both man, both. You will highly enjoy both of them, I am sure of it. If you are tight on money though, uhhh. Hard decision there.

Flip a coin.

Both are excelent games that you should get at some point, but if only one must triumph now, I say go with Mass Effect.

Dude... look at the prices! You might not be able to get them this cheap again..

i havent played either, but if you're interested in both (and can't get both) i'd get the cheaper one>

or put the money towards left 4 dead.

sorry i only have two. none to spare at the moment

Let me put it this way. Mass Effect was a game with lots of glitches and bugs and some very unexciting and plain cookie cutter side missions. The story was good and had some great cinematic moments and while the choices you can make rarely ever changes an outcome it still makes you feel like what you say matters.

Bioshock on the other hand is one of the single greatest gaming experiances I've had this gen. It's atmosphere is second to none. The story is amazing with an increadible twist. The powers and guns at your disposable make you feel like a god and are set up to give you some moments of "wow! cool! Did I just do that?". The environment is rich for you to take advantage of it. It's just an increadible experiance. I recommend everyone play that game while I would suggest only huge WRPG buffs check out Mass Effect. Your choice though. ;)