Power. That's coming from someone who went from a 320D to a 335D and I can't praise itenough! She is f****n saaaahweeeeet when I feel adventurous and even when I don't I hit cruise control and just relax. I know if I need to, I hit the pedal and I'm gone like the wind and it's such a good feeling to know you have the power there if needs be incase some cocky jocky gives it large in his type R or whatever it is (excludes the AMG/Porsche cars here lol).
Like people have already said, lot of the add ons can be retrofitted which wuold make for a much better combo. As makkan has already said, you can fit options into a 535D but you can't retrofit an engine into a 520D unless you are off the rocker lol.
Again, just can't stress enough just how amazing the 35D is in comparison to the 2.0D..