Hi there. I got out of an E220 CDi Avantgarde into my 530D GT & have been delighted so far (nearly 4 months with the 5er).
Mine came with 20 inch wheels via the dynamic pack. I find the ride to be very smooth on well maintained roads. On poor quality roads, it can be sometimes harsh but not much harsher than what I experienced in the Merc. You do feel deep potholes & the vibrations felt are slightly amplified due to the panoramic roof which means that the body is slightly less rigid than a full piece roof without the glass.
I would agree with the other post which suggests a long test drive to fully evaluate for yourself.
I would strongly recommend the rear reversing camera. I have a tight car park space at work & find this to be a huge advantage.
Good luck. I'm sure you won't be disappointed!