Mass Effect. It drags occasionally due to repetitive sidequests, but I still say it's the best game on the console.
... on that note, while I feel that BioShock is a completely solid game, I simply don't understand the sheer amount of jizzing over it. Outside of the amazingly fantastic setting, I didn't find anything else to be overly remarkable about it. I don't know, perhaps I let all the hype from people raise my expectations to high, but I wasn't wowed by the game's engine, story, or anything else, aside from the aforementioned setting.
Point is, for the OP, you can get both easily... but I can say that I've played through ME 4 times, and I've gone through BioShock once.