I would go for the GTS - absolut stunning car....and unless you want a interior closer looking to a Panamera/Cayenne designwise in the 991. Engine wise there will be a couple of HP difference and yes the 2nd generation PDK might be faster - but despite long waiting times mostsurely you have to consider the price difference of a nicely specced 991 vs a actual GTS...

I decided mainly engine wise for the RS - I wanted a Mezger-engine and no DFI and 3.8 Liter 450hp sounded just fine