I agree with your comments on the Panamera diesel, which is too expensive and I would take a 740d x-drive over it any day.
Not having 4wd here is not that useful in winter time (past two winters were a nightmare). Yes, I have M+S tyres on my 3er but (i) it does not save the day if you have to make a hillstart and (ii) I get around still quite alright, but my wife can't / doesn't dare to drive the car anymore.
So a Panamera, which is too expensive in comparison to the competitors, without 4wd is a no go.
The Cayenne Diesel on the other hand is another story (for me and my wife). Honestly, I do not need a SUV and for my future replacement car (which according to my wife is not to happen in the near future) I would prefer a 530d X-drive Touring: fast enough for a diesel, a bit bigger than what I have, very comfortable, in short: a small 7 series. On a side note: the 5 is actually remarkably close priced to a Cayenne diesel. My wife does not like its looks however (something with the lines in the hood) and would go for an SUV (she is not that the tall and appreciates a higher seating position / also, with kids in mind somewhere for the feature an SUV is convenient for putting them in their child seats I was told):
- X5 30d/40d: I have driven it, but did not like the interior (looks the same as mine and not up to date in comparisn with the 5), basically, it is an old car in need for a replacement (you feel it as well in noise insulation etc); maybe the new one
- Range Sport: very nice to drive, but I question reliability; overweight, so fuel consumption is high, performance low
- Mercedes new M: not convinced by the look (less sporty / young / dynamic than e.g. Cayenne) and after having heard one of the senior people from a taxi company rattle on during a 3 hour drive back from Frankfurt how much problems they have with their S Classes diesel which they only use for the long distances (Frankfurt, Geneva etc), I am not in the mood for Mercedes (the car actually broke down during our trip: Notlauf and Mercedes dealership could not tell what was wrong)
- VW Touareg: same basis as the Porsche, but my wife does not like the brand (yes, I know, with Porsche you pay more for the badge); I must be honest: fit & finish of the Cayenne seems better and I like the looks of the Cayenne more too.
- Q7 seems huge in comparison to the rest and my wife and I do not really like it
- Cayenne: sportier than the Mercedes, the VW and the RR Sport; more up to date than the BMW X5; I have a 911 and am happy with reliability and my dealership; although similarly priced to a 5, it has more backroom and bigger trunk (granted the Cayenne costs still a bit more and is slower); biggest range of all cars with the 100l tank (I often drive back and forth between Luxembourg, where I live and work, and Belgium for friends and family) and range is important as I do not like to stop all the time to re-fuel and from a pure psychological point of view (fuel prices are lower in Lux); I drove it and really (I mean REALLY) liked it (noise insulation, comfort with air suspension and PASM, fuel consumption during Autobahn runs, modulation of boot by moving back and forth rear bench, GPS system, ...)
As for the VW/Audi with V8, they really are much more expensive than the rest and would not be considered, so then the Cayenne is as fast or faster than the rest (with the exception of the X5 30d / X5 40d). Petrol is out of the question (40.000 km a year) as I would not want to pay the fuel price difference.
From the above, it is clear you can always argue in one direction or another, so the point is not to convince you of the Cayenne diesel, but rather show you that after having had a look at the market, the Cayenne diesel seems the best available car to some (and given that the waiting list is almost a year, I do not think I am alone on this).