Tough call, both games have their flaws and neither is a true graphics showcase IMHO.
CoD4 looks great for character models and the core play area but the physics is crap, there are invisible walls all over the place, backgrounds quickly reveal the limits of the playing level, etc.
Bioshock lighting and levels are great, but character models are iffy, animation is stiff too and I actually felt the engine ran a little sluggish for what it was displaying - some levels and rooms were pretty weak too. Some levels also had poor backgrounds too.
However, Bioshock leans heabily to combat but has weaker FPS and the engine, while nicely used for the Art Deco style, doesn't deliver the experience for combat I think it should. And while CoD4 I feel has plenty of little graphical weaknesses, it does achieve 60fps and it feels like any compromises were made to maintain that.
So CoD4... just.
Try to be reasonable... its easier than you think...
