The face/spoke concavity of both 8.5" wheel look identical and of course each accommodates the same range of tire sizes. The only difference is the 8mm delta in offset (ET42 being more flush to the front fenders). Cayman/Boxster drivers looking for 8.5" front wheels can choose between the two offsets depending on the look they are going for, taking tire size/model and whether or not negative camber is dialed in into consideration. It is quite difficult to see the difference in offset in photos.

Here is a photo of the 18x8.5" ET50 with 245/40-18 RE-71R tires:


Here is a photo of the 18x8.5" ET42 with the same 245/40-18 RE-71R tires and approx ~2º of camber
* If there wasn't any negative camber dialed in, the tire would sit slightly closer to the front fenders


- Ryan