It really depends on your driving and what you want to get out of the car. ED will
be more economical, but the benefit will really depend on the driving that you do. I wouldn't expect them to be that different in a steady cruise, but the start-stop should make a more significant differnce in city driving.
The 320d is a fine car in whatever spec you choose. Okay it's not blindingly quick, but it is more than enough for everyday driving. The economy is excellent and even on my old E46 320d I would see mid to high 50s without trouble on a long cruise (and even 60mpg if you were careful).
The six cylinders are a bit of a different animal. The six cylinder engine means that they are noticably smoother and more refined, but don't make the mistake of thinking the 320d a tractor as it is far from agricultural. Yes, the economy suffers, but then you do have a heap more go under your right foot.
By way of comparison my E36 320d was giving a real 43mpg on my normal commute of about 50% motorway and 50% city (by time not mileage). The E91 330d is currently returning just shy of 38mpg and is slowly climbing as it gets some miles under its wheels.
Try them both an you'll be sure of your decision.
Spot the difference: