Actually, as most members has put it, they cayman and 911 are both different breeds. I've had the pleasure of driving both as I was testing cars for a car magazine two years ago. They cayman, as most motor journalists put it sits in between the boxster and the carrera 911, which if u have watched past episodes of top gear, your would have heard jeremy clarkson calling the cayman the cockster. The charts prove the same, the cayman's performance figures sit directly in between the two as well.
Both cars can be used as a daily driver, that's the beauty of porsche, it's never complicated. If you get one with the sports chrono package, it comes with launch control that you can keep using over and over, and it never burns out.
Sure the 911 has more room at the rear but no one ever sits in there and it is unfair to compare the two siblings. One is an older and has a longer heritage laid down in Motorsport history. Porsche also says as long. There was a Ferrari around, there will always be a 911 to challenge it.
The 911 used to be a bit of handful due to the balance of the car, with a heavy engine in the rear it was a nightmare to drive as the back would break loose in corners. Drive it without respect and it'll end up killing you. Come to think of it, it was oretty much the same with older ferraris and lamborghinis. With the new 911s, it comes with so much technology that holds the car in place and one does not need to worry about over cooking it around the bends. It's also the flagship model of the Porsche stable.
The cayman is no slouch either, however you can't help but feel that you've bought the cheaper model if a 911 pulls up beside you at a set of lights. If you do get the cayman, do go for the S, you won't regret it.