530d will open a lot more doors as well as having more power all round and not much difference in mpg theres probably 10 530's to every one 525d so finding one is a hell of a lot easier

I cant add to much as my knowledge is on petrols but im sure diesel owners will chime in soon

But what i will say is you wont regret it its a big step up from the e36 and thats a great car in its own right iv loved every minute of ownership with my e39 must have it 3 or 4 years and no intention of changing