Seems you have done pretty well on the research; and know what most people will point out to you!

Suppose you need to sit down and work out the figures - taking into account all factors; premium to buy a 520d vs the offset in road tax of the different engines - 280 vs 750 - say 500 quid....if you are paying a 1k premium for the benefit of the 520d and the 280 tax, then thats two years worth of road tax in the 525i.
If you are doing low enough miles, is it worth getting the 520d - again a few calculations; mpg vs the cost if a turbo goes, etc

Have you test driven one of each?

Edit: Cummins, formerly of Hilltown?