Are you guys just looking at cost per mile as you drive, or the possible long term use and longevity to injection systems and cleaner EGR systems and the like? Remember a lot of the injection equipment makers also supply additives to enhance the life of injection equipment. Companies like Delphi and Stanadyne are in this group. Many in the industrial segment use product like Forte and Coundown as a matter of course, to give engine longevity and reduce long term servicing/maintenance costs. The additive package in premium fuels is not to be viewed just on a short term basis.
Cetane is important to efficient fuel burn, so getting fuel with higher cetane rating (around 55) will help smooth out the flame burn and make for better combustion efficiency. This has an effect on soot levels and particulate matter. Premium fuels typically show a lower Bosch smoke level.
We feel the change in many performance diesels, they are quieter for one thing, as diesel combustion knock is less.
Some engines do return more mpg on premium fuels, but short term costs are most likely more expensive. But imagine down the line, when internal parts are cleaner, less coke in the ports and valves, EGR valve is typically cleaner with premium fuels and/or additives. Even the oil contamination between services, is a factor for why a premium fuel could be worth using.
If you are trading the car after a few thousand miles then take your pick. But if you intend to do 200,000 miles with the engine, then you may want to consider the way your diesel is used and how that could effect longevity and maintenance costs long term. Simply another 40k miles from the injectors could cover the fuel cost difference.
It's all a gamble anyway, but some do see added benefits to using a premium fuel. Just widen out the thinking, not just the cost per mile today.