I would say Call of Duty. It's more intense and large-scale. The fact that there were people who actually fought and died in these situations makes it even more powerful.
I've played CoD4, CoD5 and Halo. I used to play them all a lot for long periods at a time where none leave the disk tray, but more recently I've gone back to Halo because of all the new content in matchmaking and things such as Double-XP playlists, though I switch them regularly now depending on what my friends play.