You know, after reading all of the threads that were being made a few weeks prior to Halo 3 being released, I'm surprised at the response.
While they're both FPS games, they're very different in their style. CoD4 is very fast paced and is for those who like the "realistic" feel of combat.
Halo is for those who like a style where getting shot at doesn't mean you're immediately dead and have time to do something about it.
Truthfully, I enjoy both games, but my online experience mainly sticks with Halo 3. The group I play with generally sticks to that, plus my roommate and I play on the same television, which takes CoD4 out of the equation because there is no split screen when it comes to online play. Just like that, Halo 3 becomes the main choice.
Both games are still popular. Nights that I log onto Halo 3, I still find nights where there's 300,000 to 500,000 people playing it (Whenever the server works enough to actually give a population), so neither game is hurting for people.
Eventually, something else will come out that will trump both of them. It's just how the business works nowadays. Developers will always work hard to knock off the current "best game" that everyone is playing, which provides a better experience for everyone. There will always be that "next big thing" right around the corner....
And that's why I love being a gamer in this generation.