I will say that for looks the 18" Cayman S wheel is my personal favorite:
It seems to look a little "tougher" and the tire sidewall is shorter to account for the larger diameter of the wheel. To some people those particular ratios look better.
For 17" my favorite wheel is the Boxster S twist:
That is another "muscular" looking wheel but is heavier than the 996 Carrera Twist which is a common wheel for spec boxster:
I got these images from here:
944racing