There is a massive difference between the two, if you enjoy the power of a Cooper S a Cooper D probably won't excite. I have driven the 120D and that's fairly swift, though I was sick of changing gear all the time and would probably opt for the auto. Saying that, I did manage to get the 120D down to 12 MPG, so enthusiastic driving will hit the fuel economy whatever you drive. Then there's the noise of course.