I'm on my second 911 (a 996 C4S Cabriolet) and they are truly fabulous cars.
That said, I was at a Porsche trackday last year and had a few laps in a Cayman S. The performance difference between the 911 Carrera and the Cayman S was very small on a track with short straights and the handling was very, very different. The Cayman was crisper and more 'natural' feeling in the bends. I was very impressed.
If I didn't need a back seat for my 6 year old, I would seriously consider a Cayman.
Trite answer, but I'd go for a drive in a couple of versions of each car and see which one floats your boat. I suspect neither will disappoint you.