This is a slaughter in favor of the Cayman. I think you need to post on Rennlist. I think more water pumpers over there and this board for the most part seems anti 996 whether people have driven one or not.
Driving both, I think the Cayman felt more modern and better quality Several years will do that. The 911 C4 definitely felt heavier which depending on what you like can be good or bad. I prefer the weight for highway road use. I also remember liking the sound of the C4 better and the way it felt on the highway was superb, but bulkier on the backroads. My 00 Boxster S was definitely more fun on the back roads but not so much on the highway at speed. I don't recall either feeling faster, quicker than the other. The spread is probably not noticeable for daily driving. Go 0-60 a 1/2 second faster, who cares?
My friend had just bought a 2007 Cayman Base and sold it quickly there after. Didn't like the power. We did take it on the track one day and it's handling and braking were superb. Bought a S shortly thereafter. I really liked the base.
Looking at, personally I think the Cayman is a better looking car than the C4 but I think it really might be one of the best looking Porsches ever built including most 911's. The 996 C4S might be the exception.
Another thing to consider is the motor is not readily available on a Cayman. If you like tinkering, it can be a hassle sometimes. The Boxster was a chore, I imagine the Cayman would be even worse. Does the Cayman have a oil dip stick? Can you look at the coolant tank easily like in a Boxster or 911?
Good luck, both awesome cars so enjoy the search.