Actually I believe one of Porsches top test drivers likes the behavior of the 18"s better than the 19"s. I pretty much agree with Eurospeed, 19's are more for looks. There are some manufactures that let big beefy tires do all the work and some that have a great suspension that actually uses what tires they have to the fullest....Porsche being one.

I'm torn between the stock Cayman S wheel and the Carrera S wheel....I like the looks of both, wish I could see the Cayman wheel in person, that would probably decide it for me either way. One thing I have noticed is that 18"s make the stock Boxster/Cayman brakes look nice and big....move up to the 19"s though and they begin to look a little on the small side even though their some of the best in the world. So if looks are important this may be something to think about also.