If you want a barebones multiplayer without too many confusing elements, get CoD4. If you prefer a world war 2 setting, and a weapon set that on average requires a lot more skill (about 2 exceptions) then get WaW. If you prefer a much more balanced multiplayer at the expense of there simply being too much clutter at times, then get MW2.
MW2 is better than CoD4 in pretty much every way.
MW2 is better than CoD4 in pretty much every way.