Depends which diesel you are talking about. 3.0sd (twin turbo diesel) beats the 4.8 hands down. Far superior real world performance performance due to extra 20% torque (at much lower and more useable revs), 40%+ better fuel economy, 40%+ longer touring range. What's more, you can drive the diesel as hard as you like and the economy only goes up by a small percentage, but drive the 4.8 hard and it guzzles fuel at an alarming rate. I've driven both back to back and for me there is no contest. The only place where the 4.8 is better is on the dragstrip and then it's only marginal. According to BMW's own figures, the 4.8 does the standing start kilometre in 26.6 seconds while the 3.0sd takes 27.3 seconds.