Theres a huge difference in ride quality between 17s and 18s IMHO, especially if the car is lowered (which it needs to be to look good on 18s).
I ran 18" on mine for about a year, but im actually back on 16s again now and enjoying the comfort and driveability much more! 18s will rub, and they will bottom out very, very quickly (and severely) on undulations in the road, never mind speed bumps.
It depends what you do with the car really - for looks/posing etc, nothing beats the 18s. For spirited driving and comfort nothing beats the 16s. 17s are a decent compromise, which is why you find most on here go for them.
-1998 916 Spider 2.0TS Lusso-
H&S Corse stage 1.
CATCAMs stage 1 cams with Savali custom ECU remap.
305mm discs with 166 4-pot callipers.
Ragazzon exhaust mid section.
Biscione Racing 2x70mm rear section.
17" Zender Siena wheels with 3mm rear spacers.
etc etc etc!