Do you mean a 5 series? If so, the 135i is $15k cheaper and is a totally different car for a different type of person. If you want a sportier feel, better handling, and are interested in a car that you can autocross or track the 135i is your car. If you are more interested in comfort, luxury and hauling more people, then the 5 is your car. MPG is not the only factor in deciding which car to buy.