I only tend to do short trips with the occasional longer outing. Have owned a 330d, a cooper D and a 2.7 diesel merc but have found that diesels take longer to warm up and the advantage on a short trip is not there to be realised.
Coupled with the EGR/ DPF issues they would be in regen mode more often, so what ecomony you did yield would be burnt away.
So Just get a V6.
I average 30mpg in my 2.2 159, yet have seen 37mpg on a run at a steady cruise. Im not heavy footed as there is no point in a car that weighs as much as a house fitted with a 180hp motor. I have a bike for more hairy moments.