To me this a no brainer; I'd go with the Cayman design edition; most magazines have reported a sub-5 second 0-60 for the Cayman, and if you really do want more speed it's easy to get.. exhaust, intake,
plenum, you name it. Heck for like 5-7 grand you can take the car to
TPC and get the Cayman up to 415 hp with 5.5 hp of boost and it would smoke the 996 Turbo.
While I like the shape of the 996, the interior, electronics, shifter and clutch feel, etc. all seem outdated and clunky to me. And I'd never own a 996 Turbo out of warranty, so if you do go that route do yourself a favor and get some coverage.