the thing is,
COD4 is a realistic modern warfare fps game
Halo3 is a sci-fi, futuristic fps game
i don't believe two games of so many differences could be compared fairly unless someone has played both intensely.
as for my opinion, i have played COD4 on the PC (Windows), but have only played little bits of Halo 1 (at a friends's house). therefore, i can only biasely say that i like COD4 better. :-)