It's a fair question, but I'd say a lot of people, myself included don't necessarily lean towards one over another. There are games where cartoony graphics work quite well, and others that a more realistic approach would suit.
Take your comparison of CoD4 vs Halo 3. CoD 4 as a modern-day shooter is better suited to a realistic style (imo) since it's a re-creation of things that we can see in our every day lives. Halo 3 on the other hand is futuristic and not at all based on things we see everyday, so having a more cartoony look to it can work quite well.
The only teeth strong enough to eat other teeth.







