A Cayman is essentially just a hardtop Boxster, so they drive very similarly. If you're really tuned into feeling chassis flex, you'll notice that the Cayman feels a bit stiffer when taking corners at 10/10. But on the streets, they'll drive nearly identically. When I was shopping, I was originally looking for a Cayman, but settled on the Boxster because of a known service history, low mileage, and price; they felt identical on test drives.
Remember that the Boxster was developed first and the Cayman didn't come out until the second generation of the Boxster. So Porsche engineered the platform as a roadster, then made it a coupe.
Unlike many convertibles which were later derived from their coupe counterparts and drove noticeably different due to the need for additional bracing in the floor.
Maybe one day I'll find myself back in a Boxster/Cayman, I'm going to miss the mid engine life once the Boxster sells... The handling is far superior to my 911's, though the AWD puts it into a class of its own.