Well it depends, the 540 is a better daily driver. More torque off the line, a super-smooth V8, lower maintence bills if something breaks (assuming the motor is an alusil, wouldn't buy a Nikasil - see www.530i.org for more info if needed). The M5 is the better sports car. Quicker if you are willing to rev it, handles better, sportier. These are the things you need to think about. Also to get a nice 540 will be sheaper than a nice M5. Good luck.