If you haven't played Halo 1 or 2, then Halo 3 is far better than COD4 (in my opinion). I wasn't that impressed by halo 3, as it was just more of the same.
COD4 is good as it takes the series in a new direction.
Halo 3 is good if you have 4 controllers and 4 mates over because you can still play online...
TBH games are only worth buying if they have very decent multiplayer as thats what gives them replayability.
Halo wars looks like it could be good although its going to be competing with tom clancies end war which looks like its going to be a incredible with its innovative voice control system. as stated above its an RTS (real time strategy) game not an FPS (first person shooter) like halo and cod4.