"From the smallest necessity to the highest religious abstraction, from the wheel to the skyscraper, everything we are and everything we have comes from one attribute of man - the function of his reasoning mind." ~ Ayn Rand
Bioshock didn't impress me, I was already dazzled by Dead Space I guess so Bioshock just seemed mediocre to me.
Borderlands however is great, I got a little bored with it but with co-op it has fantastic potential.
If you played Bioshock 1, you got all you need. Bioshock 2 just doesn't compare really. Dead Space was my favorite game in a long time, but thats not to say Borderlands isn't worth a try. Borderlands was honestly the most straight-up fun game I've played in a while. It certainly has its flaws, but its just great visually and the gameplay is only designed to be ridiculously fun. I'd go for Dead Space over it, as thats just a game you need to experience, but Borderlands is also a great choice.
I've only played Borderlands, so I can't say if it's better than Bioshock 2, but I can say that I've thoroughly enjoyed it so far. Keep in mind that you should play it with friends to get the most fun out of it.