The 2 games do not compare in the slightest. Get both?
Both are good but on different levels. I have Bioshock and so far I love it but I refuse to play any more of it until I build myself a new rig that will run well with the Graphics maxed out.
Haven't bought CoD4, see above reason.
I may get CoD4 on the 360 though just so that I can kick my manager's arse on Live.