I can't deny that Halo 3 has a ridiculous amount of content and options in it, but it's still Halo. It honestly feels exactly like Halo 2, at least to me. That's not really a bad thing, but Call of Duty 4 just feels like what next-gen should be. Its graphics are fucking mindblowing, the gameplay is solid, traditional COD, but transposed in the modern era, it feels fresh. The guns all feel (rumble!) and sound realistic, the speed of game is quicker yet the controller works surprisingly well (and the auto-aim never seems too ridiculous), and the sheer madness of battle has never been relayed through a TV screen so well. And I haven't even played the single-player.
The create your own class system is great, and allows you to have different guns and perks set up for different modes. The sheer amount of weapons, perks, and rewards are just awesome. Gameplay-wise, COD4 is a step above Halo 3. Also, the matchmaking in the beta, if not better, definitely quicker than what is offered in Halo 3. The lack of splitscreen online is very disappointing though.
This is not just me talking either. My two buddies who played the COD4 beta over my house (and also one of their houses on an HDTV

) were fucking amazed. We couldn't stop playing, even though we had to pass the controller around.