The real life performance gain from a 4S to a GTS is minimal, even if the GTS may be in fact the sportier model, especially regarding the chassis. However if one needs to ask if PDCC, a major performance enhancing option, is necessary, I don't know if switching from a 4S to a GTS really makes sense.
I would not hesitate a second to recommend the GTS over the 4S if someone wants to make a new car buying decision, the GTS is clearly the much better deal than the 4S and I have to admit that I don't get it why some people are still opting for the 4S instead because you can order this car in any color and ride comfort is really not much different but you get all those performance enhancing goodies which make the GTS more fun to drive.
Switching from a 4S to the GTS is a different story though. If you can have the lease deal for a one year lease duration only and/or if you can switch to the facelift as soon as it comes out (there are certain lease contracts which may allow that, depending on the region you live in), I would do it. Otherwise, you are going to end up with an "old" car in very likely one year from now. The only reason I couldn't wait with my switch to the PTS was the lease contract I had and the fact that I need to get rid of the Carrera GTS Cab because it just sits around and eats money. I also have been told that the facelift will not include the Turbo S from day one and that the power bump of the regular Turbo will be 20 hp or so at max, not really the performance enhancement I was hoping for. Of course the next generation Turbo S is expected to have 570 hp minimum but I couldn't wait another 18-24 months. Impossible.
I would only switch from the 4S to the GTS if the deal is so good that you can't pass on it. Unless of course you don't care and you just want to have the latest tech and style. Then, wait for the facelift.
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RC (Germany) - Rennteam Editor Porsche Panamera Turbo S (June 2012 delivery), Porsche 997 Carrera GTS Cabriolet PDK, BMW X5M, Mercedes C63 AMG Coupe PP/DP, Mini Cooper S Countryman All4