Originally Posted by Brosef
putting aside the mezger block, in my mind there's no question that the M3 engine wins in overall power, smoothness, willingness to rev, flatness of torque curve / tractability, and of course sound. I would also say it wins in reliability when you consider the number of engines that blew up (not literally) due to faulty intermediate shafts on early 997 cars (mostly 2005 but some 2006 cars). however, the Porsche flat six wins in fuel efficiency, especially the DI motors.
Could not have said it any better. This is dead on accurate as can be.