I never buy cars new but I understand somebody has to if I'm going to enjoy my depreciation savings. Since you seem to be leaning toward a new Cayman, just wanted to make sure you're aware Porsche is about to introduce a completely new model. It looks like its going to be a pretty big step up so I would definitely wait for the 2013.
As far as the engine being in the wrong place in the 911, please, thats such an old saw. The ideal configuration for any sportscar is clearly mid-engine, but that design obviously has its compromises for a street car. Therefore 99% of cars are front engine which also trades performance for practicality. Porsche's racing success over decades with the 911 more than validates the design.
When it comes down to it, most of us aren't really going to notice the ultimate performance differences between a MR or RR setup, so it really comes down to which you prefer. You're going to have drive both to make that decision.