Well, atleast the Cayman looks the part of a classic Porsche. There are still plenty of people who think that Porsche ruined the 911 after the switch from the 993 generation to that of the 996, but I disagree. I've driven a few differnt Porsches in the few years I've been around, and I like most of the newer models because they are easier to drive, and easier to live with on a day-to-day baisis.
Granted, I havent driven the Cayman, nor have I driven a 997 911. But speaking with presumptions based uppon drives I have had in Boxsters and 996 911s, there isnt going to be an outstanding difference between the two.
I look at it this way: If I want to save some money by buying a 911 that isnt a 911, the Cayman is the way to do it. For the $50-60K USD, you are getting a pretty damn-good 911 knock-off, but the problem is is that it is not a 911. It may look the part, but the engine is further forward, smaller, and there are "only" two seats.
Both Car and Driver and Motor Trend attempted to tackle the issue of 911 vs Cayman a few issues back, and both had said that it does come down to how much money you want to spend, and how much "functionality" you would want in your Porsche. On paper, just looking at the performance figures, the differences between a Cayman S and regular-grade 911 are quite minimal, and price differences are quite small.
Again, if I'm buying a Porsche, I'm going to buy the Porsche that has defined the company for the last 40 years, the 911. There is just something about the car, its kinda like saying once again that it has a soul. The car has expirience by comparison to the Cayman, and not to deminish it, but it is still a young-gun in a class of veterans dominated by cars like the 911, Corvette, Z4 M, etc. The 911 is a better buy, simply put, because it has expirience.