The 997 Turbo S Cab is a rocket. The 991 Carrera 4S Cab is not slow by any means and the improved chassis is fun to drive but if somebody is addicted to pure power, the Turbo S Cab. would be my choice.
Gregor, just ask yourself what you want: The 991 Carrera 4S Cab is the newer model, advanced technology, better chassis and a better exhaust sound with the PSE. It won't get old for the next couple of years. It is still much faster than your old 996 C4 and since you never owned a 911 Turbo, you may not miss the power at all.
The 997 Turbo S Cab is a beast in disguise, this car is so fast, it is just amazing. Once you get addicted to the boost, you never want something else. You can't really go wrong with this one but just be aware that maintenance cost is higher (also repairs outside the warranty, you should get a car from a certified dealer only), the exhaust sound is not really great (which can be a plus in a country where people react to too much noise pretty negatively, like Germany or Switzerland) and as soon as the 991 Turbo Cab comes out, your car is "outdated".
Difficult choice, I know but for the sake of the newer model (and better chassis) and for the sake of keeping the driver's license , I would probably choose the 991 C4S Cab over the 997 Turbo S Cab in Switzerland.
The final choice is yours and trust me, I get your dilemma very very well.
P.S.: If you go for the 997 Turbo S Cab, make sure the car has not been tuned before. Many Turbo owners play around with cheap chip tuning and this may affect the durability of the engine and drivetrain in a very serious way.
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RC (Germany) - Rennteam Editor Porsche Panamera Turbo S, BMW X5M, Mercedes C63 AMG Coupe PP/DP, Mini Cooper S Countryman All4