i've seen halo 3 being played and it was thoroughly disappointing. its exactly like the other two halo games. there isn't really anything different. you just have different crappier guns, and you're fighting some slightly differently tailored aliens...whoop di doo. there's really nothing to it except to run around and shoot stuff. hell..you could probably just run through half of it and make the story line advance that way.
bioshock actually throws in ethics, new fun weaponry, awesome genetic changes that make you amazingly powerful, it has graphics superior to halo 3, and it has several different endings depending on what you do. there are tons of hilarious things that happen in it too. for example...an enemy might try to throw a fireball at you, and miss, lighting a pool of oil on fire. and then, amidst the chaos, the enemy might teleport straight into the pool of fire and burn himself to death. that is freakin awesome. you never see aliens burn to death in halo, do you? i think not.