Good point B w.
Allow me to explain my thoughts.
According to Wikipedia dictionary there are numerous styling, equipment and technical differences between the two vehicles.
And as you pointed out they share the same chasis, but some further reading lead me to understand that Audi was on the same band wagon.
Wikipedia says that "The Q7 is available now, sharing its platform with the VW Touareg and Porsche Cayenne. Based on the Pikes Peak quattro concept car, the Q7 is designed more for on-road use than its Volkswagen cousin. It lacks the Touareg's locking differentials and low-range transfer case but gains a third-row seating option."
Since they all have the same chasis does that make them all the same. I should hope not. What makes them different?
The interior design, colors, wheels, options, and engines.
The Cayenne shares only its base engine with the Touareg, and Porsche's version is substantially modified.
With that, I didn't go in that depth as I wanted, but it seems to me that the Porsche would be on a higher standard...higher than the rest. VW is trying to expand their model line and who other to explore with other than Porsche...who makes very well high end sports coupes/convertibles. Since VW didn't have anything special to put in their engines, unlike Porsche who has the riduculous Carrera GT/911 for example to put into their SUV.
VW just found and modified their engines and the largest probably coming from their luxury sedan that tries to compete with BMWs 7-Series. Yeah right. Good luck with that.
Which leaves me to not forget about Audi's Q7. Nissan has some credibilty when it comes to engines. The 350Z and the G35 are pretty decent. They probably just used those engines with modifications as well as design interior, colors and option from their ever failing model of the Moreno, in which I wouldn't consider in a heart-beat as a personal SUV. Now, the Infinity FX, which owned by Nissan....is a step up IMO from the Moreno and similar in the bubble/oval look. I have driven one of these, the FX35 unfortunately but I was rather impressed. They used the G35s exaust and probably the engine as well. But one step closer to the top by modeling after Porsche and VW.
Audi is trying to get a new look for their SUVs, and who other to join in with than Porsche and VW. With the same bubble oval look, like Porsche, and VW...Audi is slowly learning. Audi is a good company too, competing with BMW helps. Their A4, S4, A6, and A8 are very nice places to start for Audi. I think they have may have something and perhaps overtake VW in this run for 2nd.
Annnnd, we also forgot to mention other models that aren't based on the same chasis. Mercedes-Benz M-Class, Lexus RX, Volvo XC90, BMWs X5, Land Rover Range Rover, Land Rover LR3, Escalade, and Denali.
But I won't go into that, because it isn't relevant to the discussion.
In retrospect, I think it all depends on each individuals persons tastes. I know that everybody is different, and from my view point, I would want more of a sporty SUV. Porsche grabs my attention first, but VW is trying to attract a different audience. Like for example. A good friend (in Seattle)...we were talking about cars and she wants something cute. Well minivans were in the converstaion and gradually switched to SUVs. She immediately said the VW Touareg. I rolled my eyes, being somewhat biased to the X5 4.8is told her my thoughts...and she begged to differ. VW has a different approach in trying to get luxury (unlike Lexus RX Series) and sportiness (unlike Porsche or BMW). Kind of in the middle scale.
So yeah, it all depends on what you want and what you are looking for in a SUV that appeals to you in your many choices of SUVs out there.