The people say Battlefield 3 is better. Graphics? Not Call of Duty?
The next generation of Frostbite will debut with Battlefield 3. It will take full advantage of the DirectX 11 API and 64-bit processors, with no support for DirectX 9. It will also feature enhanced in-game destruction with Destruction 3.0, creating more refined physics than its predecessor. Additionally an important distinction on its naming is that Frostbite 2 is not called Frostbite 2.0.
DICE has given several presentations on advances in their rendering technology
Here is the Modern Warfare 3 Engine
The latest version of the IW engine is the MW3 engine, which is a modified version of the IW 4.0 engine. The engine of the future title Call of Duty: Modern Warfare 3 is named MW3 Engine because the teams behind the game are now Infinity Ward and Sledgehammer Games . It is speculated that the next version of the engine will not feature destructible environment. Despite it being available at the time, Treyarch continued to use the older version of the IW engine in Call of Duty Black Ops due to their familiarity with IW 3.0.
Personally I think Modern Warfare 3 is much better because it has improved.
Battlefield 3 was already like that just had a little tweak.