We opted for the 528 due to known issues with the 535 DI fuel system. Not as much fun, but great gas mileage and 100% reliable (so far). I don't know how much the 'x' adds in weight and performance penalty.
I would be leery of owning that generation of the 535 out of power train warranty.
Any out of warranty work is going to be expensive, and little things like the CD player and iDrive electronics caused recurring problems for some people that cost big $$$$.
Great driving cars though. They just feel right. And the torque of the turbo motor in the 535 is awesome for everyday driving.