I kinda used to think that, but I honestly think that Halo 3 takes more skill due to an emphasis on aiming and memorization on map and weapon placement and strategy. Also, working as a team is emphasized in Halo more since in CoD you can decimate an entire team by yourself, due to being less balanced.
However, Halo has always been more of a custom games type game for me, and less competitive. So I can see where you might get CoD being more hardcore. I also feel more into the game in CoD as well, with my Turtlebeaches being such a bigger help in that game (e.g. I feel all pro ^_^)