CoD and Halo are completely different. The biggest similarity is simply that they're both First Person Shooters. Not much else.
Halo is pretty fun and has all kinds of variety and sci-fi stuff. Which I've always loved. But CoD has the fast-paced strategic multiplayer you'll always find yourself going back to enjoy.
I've played Halo on XBox and Mac (The original version of Halo was created for Mac, most hardcore fans don't know this =P) and I enjoyed them. Especially the second one, and Halo Reach.
I've played all of the CoD and the first MoH game that fits into the series. And I've honestly enjoyed them more then any other mainstream FPS next to Operation Flashpoint and Metro 2033.
Honestly I enjoy CoD more then Halo. Modern Warfare for Wii is my most favorite FPS game I own. But Halo will always have a nice place in my heart for me.