Why not buy my 11,000 mile SRT-6 with the 6 year 100,000 mile warranty for less than half the price of the Cayman? I'll even throw in the never-used SRT-6 luggage.
We have oodles of the little Porsches in stock (parkplacetexas.com) but I don't see any reason to do one of them over the SRT.
The Cayman S is slightly lighter but has no torque to speak of. It's a Porsche. It's got to scream through corners because it doesn't have the grunt to do the job.
As far as ultimate handling goes, the Cayman S will pull a little higher constant G but holds no tangible advantage when it comes to transitions between turns such as a slalom or twisty road. When things do straighten out, the SRT will walk right on by.
If you have got to own a manual transmission, do the Cayman S. If it's curb appeal you're looking for, the cars are pretty well even.
For performance, I'd go with the SRT-6 every time.

Just for a feather in the cap, I'll tell you that I haven't met a stock CTS-V or even XLR-V that can hang with the SRT. Some have tried. All have failed.