BioShock is a wonderful game. The story, the music, the voice acting, the lighting, the water... it's really gorgeous - if such a thing can be said about such a twisted place like Rapture
I just beat it about a week ago and unlocked 46/50 of the achievements (took my time, was busy!)... I need to play it on hard and save all the sisters this time to unlock most of the rest
Hands-down though, I don't think it's fair to compare Halo to BioShock. While BS is an FPS, it's not very traditional and the replay value is very low. There's no multi-player (MP) and it can get repetitive. For a scary game, it's great (although I admit that wears off very quickly). I heard FEAR and Jericho are pretty good horror games.
Halo? It has a HUGE fan-base that continue to find new things to do in the game. Sure you can play the game normally, and play the MP normally, but then there's the glitching side and eventually if you're really into MP you'll probably get exposed to new tricks or ways to help your gameplay. People continue to play H2 till this day. H3 will likely enjoy the same amount of gameplay and more, especially with the in-game video recording and Forge. Also, it's competitive.
I'd compare H3 with Unreal Tournament, Rainbow Six, and Gears of War as far as FPSs go. BioShock, while an FPS, is somewhat different to me.