V8's sound nice and have a nice bit of kick to them, personally I think you need to own a V8 at least once in your life and with the way fuel prices are going now is as good a time as ever. I miss my V8 beemer once in a while, mostly on the motorway.
530i is a nice car though, will be more economical and has better steering.
Bar the small capacity diesel's none of the E39's are very economical by modern standards so its much of a muchness.
That does not make much sense to me, I would think the lighter car with the smaller engine would be more economical when cruising seeing they have the same rolling resistance. When you are at a certain speed you don't need a lot of power to maintain it.