I have the right to bitch about a company changing maps and guns and calling it a brand new game. If you buy MW3, you waste money. End of.
The two are completely different types of FPS. COD is more arcade-like. BF3 will be more about immersion and realism. They can't be compared on the same scale. If you just wanna run in and shoot a shitload of rounds everywhere, get into the action quickly, and speed through games, COD is for you. If you want to employ tactics, use cover, and use your ammunition sparingly, go play BF3.