I made the change from a 64 reg B8.5 to a B9 S4 a couple of months ago. B9 slightly more economic, surprisingly it seems quite a lot quicker, 2017 B9 S4 and S5 I tried didn't seem as quick and my new car has only done 2400 miles. Low speed ride on 19 inch wheels not quite as god as previous car on 18 inch wheels, but not too much in it, I would have preferred 18 inch wheels but 19 standard now. Virtual cockpit good, flat bottomed steering wheel I'm not keen on, the last car was round, which I prefer. Massage function on seats I don't use and I preferred the old car seats with alcantara. Ventilation not as good as you can't have individual vent to face and screen. Overall refinement better, modes seem more appropriate. B8 has a little more internal storage space, but B9 has more passenger space, rear passengers seem more comfortable. Both are very good cars, later DGS boxes on B8.5 should be OK, but have been a few failures. I would ensure you have a warranty just in case.