I agree with everyone else, these cars are of two different classes. Porsche doesn't compete with BMW, BMW competes with Lexus, and Mercedez-Benz, of which, the cars are composed of luxury as well as performance. Note the different lines of cars as well, the 3 series is the C-class benz, whereas the 7-series is similiar to the s-class (or maybe even the e-class). AMG to M, CSL, hamman and so on.
Within BMW's comptetitors there are competeting cars for each model of that make. Porsche doesn't really compare to the BMW, save the Z4/Z3 to the Boxster. Porsche's line of "performance" vehicles are simply that, performance vehicles. The lowest of this line matches the highest of BMW's, such as the Ms. It really is not proper to compare the two because they are totally not similiar classes. They are both good and bad in their own ways, neither is better.