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. :-)
I've played all the halo's and all of the COD's (well I only played a little of COD2), and I gotta say hands down, Call of Duty wins. In my opinion at least, it's a whole lot funner. (Multiplayer and Single Player).
i like the higher life concept in halo way better, because in cod4 you die way too fast to do anything (even tho many like this concept). ***Halo 1-3 for me***