I went for the 18" wheels. I'm very happy with this, it's the perfect choice for me I think, but if I was forced to choose 19" or 17" instead, I think I would go for the 17" for the comfort factor.
That said, I don't notice much of a difference between my 17" winter tyres, which should even have a "softer" rubber in them, and my 18" summer wheels... The driving is great anyways.
****** - just uploaded a couple of photos of mine that I picked up yesterday. S Line with 19s.The bigger the better IMO, ride is fine so far but I have only done 30 miles. Ride is much comfier than my A4 S Line that's for sure!Don't forget to order that poxy spare wheel