I have the 18" on mine and they can be a bit harsh at times, but not enough to bother me.
They are probably more responsive in turns due to the shorter sidewalls (all things being equal), but they are also easier to damage here in pothole land (my biggest concern)
Not sure about the weight difference between the 2, but more unsprung weight in general is harder to accelerate and to damp on rougher roads; if VW tuned the shock damping for the 18"s, that won't be a big issue and the ACC (which we can't get in the States) may also make that a non issue.
I did not order my car, so I did not give it a lot of thought, but the test drive did not lead me to believe that the 18's were something to avoid.