The differences are simple to point out:
1. 991 Turbo S feels much lighter and more nimble than the Panamera Turbo S, even when driving straight and/or at slower speeds only
2. The steering oft the Turbo S feels more direct and more "connected" (can't say if the car had the "Plus" steering but my PTS had)
3. The ride of the 991 Turbo S feels less comfy (chassis is quite connected and "moving" less) but actually the comfort is better since the dampers seem to work less (it is difficult to explain)
Overall, the 991 Turbo S feels like a smaller, much lighter car but I cannot deny that the PTS feels nimble for it's size and weight.
I got rid of my Panamera Turbo S because it was actually getting on my nerves driving SLOW (funny, isn't it?). It felt like a Mercedes S class when driving straight and slow, something I really hated. Only when I was really fast and pushing it, the PTS felt almost like a 911 but I want to have that 911 feeling all the time, not only when pushing it. This is why the PTS started to annoy me (and by annoying, I really MEAN it). When driving slow and straight (most of the time in the city), the ride was just too disconnected, too artificial, too comfy, whatever. Especially the steering was to blame and in winter time, I could barely "feel" the street which made the ride not only disconnected but quite dangerous because I wasn't able to "feel" the street condition.
The Panamera Turbo S is a great car for people who want a comfy ride most of the time but also want to have the option to push it hard if necessary. This just wasn't the right car for me but I get it while Porsche chose this setup. They had to keep up with the competition, comfy but sporty when needed.
If you don't need the more comfortable rear seats and luggage space of the Panamera Turbo S, the 991 Turbo S is actually even the better daily driver in my opinion.
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RC (Germany) - Rennteam Editor Porsche 991 Turbo S (Sept. 2013), Cayenne GTS (958), BMW X3 35d (2013)