I went from 18" Monzas to 17" Monzas between my rejected Mark 6 GTI and the new replacement GTI.
The ride quality on the 17"s is far superior and I talk from experience.
They also look mint against DBP paintwork, but not so sure if they work as well visually on the other colours

The 18"s are purely a vanity thing. When you are driving the car, who cares

And if you kerb them, much less of an issue.