Looks are very subjective and I just happen to be in minority in liking the Panamera looks over the F10 M5. I am not trying to justify one car over the other, just giving my insights as to what a single car enthusiast considered when choosing my car. I am a huge BMW fan (the E92 was my sixth), and huge Audi fan. This is my first Porsche.
My 2 cents:
1. The GTS drives like a bigger and more sophisticated E9X M3. The engine revs that same way but with more torque, but this is balanced with the added weight. The steering is better on M3, it is more of point and shoot, but both cars are similar in how they plant themselves to the road and inspire confidence. The weight is most noticeable when pushing through sharp S turns at over 50 or 60 MPH (there are a couple freeway on ramps that I do this on rather safely).
2. The GTS drives better than the RS4, because it seems more balanced in the turns, has more torque, and is even more sophisticated. The four drive is much more perceptible on the RS4; it understeers much more.
3. My reference for "sophistication" comes from my perception that the ride and finish of driving the GTS is of a higher quality and a more "solid" feeling than the M3 or RS4. This is case in my test drives of the 991 S as well.
4. I am really glad that I chose this car over the Turbo. For my Norcal public roads, the power is way more than enough. I have not gotten a highway speeding ticket in over 20 years and only 2 on the street during the same time so I am not a big fan of crazy speed.
5. The servo tronic steering is meh, but it was on the car off the lot.
6. The carbon fiber is AMAZING. Looks better than the M6 carbon interior.
7. The fit and finish seems worth the double of the M3/RS4 price. The little touches of metal and engraving and quality of the plastics of the buttons and temp displays.
8. The Agate gray is my favorite color.
9. I wanted the Alcantara interior, but my car was equipped with leather. I think it will be easier to maintain.
10. Initially, I thought that the 20s were too small, but after really looking the car, they are perfect for me.
11. Had it pointed out to me that all of the trim is black except for the "Porsche" and "Panamera GTS". I am getting new letters painted in black. Makes a difference to me.
12. The seats are a little firm compared to BMW and Audi. I miss the back width adjustment, but I do have upgraded seats.
13. Really, really miss the key-less entry and the multifunction steering wheel.
14. Cannot get the iPod interface to work while connected to Bluetooth, because the PCM seems to only play audio from the Bluetooth audio as a source. Cannot select playlists or tracks from the stalk on Bluetooth.
15. Sunroof could be bigger.
16. Bose is nice, but bass response can be better. The bass is really clear though.
17. The trunk is a little shallow, but definitely usable.
18. Rear view display is not as high res as BMW or Japanese cars.
19. Wheel spaces and 10 mm makes a big difference in how the car aesthetically sits. Really nice.
20. The sounds that the entire drivetrain makes is absolutely killer for the GTS. I cannot really imagine a price I would put on the whole experience for me. From startup to acceleration to downshift blips to WOT. Perfect for me in every way.
Overall, if I had ordered the car, I could not think of a more perfect vehicle for me. This one off the lot is 99% of the way there. I did a 3 year lease because I was not sure how long I was going to keep it (I average about 2.5 years per car), but I really cannot imagine a better car for my tastes and needs.
I have driven F10 M5, F13 M6, the CLS AMG, besides my own cars. They are all nice in many different ways, but I LOVE my GTS. It is the perfect blend of driving experience, quality, brand, function, and looks for me. I hope this helps!