I actually work at a dealer, I've driven both the Cayman S and 911 Carrera S (991) around the track and by far I preferred the Cayman S. I hate to admit it but its much better balance wise in the corners. The Carrera is fast, but cornering is a headache some times with the amount of maintenance throttle you have to do to keep it in check, don't even get me started on the lift-off oversteer. I was a huge advocate of the RR setup on the 911 for a while, but being a long time sim-racer and a track newbie with only about 10 hours under my belt, I just wasn't used to the 911. My point being is that it kinda sucks when you have to fight the car, even if you are a very skilled driver! In the Cayman the noobs are fast and the pro's are even faster, its just a very easy and fast car to drive.
On the 911 Turbo, I haven't driven it on the track but I've driven it plenty on the road. The speed is mind boggling and the noise is a mixture of Satan and a fighter jet, but my personal preference will always be the NA GT3. The Turbo/Turbo S sounds fantastic, but you haven't lived until you've heard a GT3 RS take off down the highway! It turns your brain to butter. I'm very privileged that I got the opportunity to extensively drive a 2011 GT3 RS a few months ago and believe me it's everything you think it is!
EDIT: "My point being is that it kinda sucks when you have to fight the car, even if you are a very skilled driver!" I didnt mean to imply that I think I'm a skilled driver, I meant for other people! Our dealers race team used a GT3 RSR and did okay, but now they're racing an Audi R8 and have been finishing much higher in the grid.