For me the 18" wheels are really good.
Our roads here in general are fairly smooth and non-abrasive, made of compressed asphalt, but there can be sharp little bumps or pot-holes and the steel lids of sewers etc. are never quite at the road surface level. Nothing big, but the sort of thing that jars. Of course frequent construction zones can be quite rough.
With their slightly wider sidewalls the 18"s soak up some of that, and you are less likely to damage the sidewall if a larger shock is encountered.
I know lots of members like the looks of the 19"s or 20"s but that is a non-issue for me--I'd rather have the smoother ride.
Of course you could also add to the price and get the adaptive suspension which should make the larger wheels feel better.
Whatever you choose Porsche's suspension management is renowned for a firm but compliant ride. Everybody who has ridden in my car (2017 Boxster) comments on how smooth it feels, and it really does 'stick to the road like glue'.