Maintenance is key! My wifes 420 has about 77k on it and no problems except the O2 sensor is bad. The 320 is sweet machine but the 420 is a mean machine. MB has been making v8's for over 30 years so they have it down pretty good. BMW has only been doing it for about 10 and they seem to be reliable too. Therefore with triple the time under their belt, it is proven technology.