One of these is a 4,000 + pound all wheel drive SUV and the other is a relatively small passenger car equipped with either rear wheel drive or all wheel drive. Comparing the two is difficult since they are entirely different classes of vehicles.
The one thing these two cars have in common is that they're both built by VW. VW has owned Audi for many years; the two cars are very much related. In fact, the five cylinder engines going into Passats and Jettas today are based on the 2.2 litre five cylinder Audi engine that I had and loved in my 1987 Audi 5000 S. On the highway, that 2.2 litre engine would get up and run easily with big Cadillac and Mercedes Benz cars, while getting a lot better gas mileage.
The Cayenne is related to the VW Touareg only in general shape, although the two were developed by Audi and Porsche jointly. Most dimensions are not the same, and the engines, transmissions, and running gear in the two vehicles are totally unrelated. The 3.7 litre V6 in the 2008 Cayenne base model provides 290 HP; the normally aspirated 4.8 litre V8 generated 385 HP, and the twin turbocharged version of that V8 kicks out an even 500 HP.
Porsches are not inexpensive to own; however, they don't cost near what many people claim they do. My friends with BMWs tell me their cars have reliability issues. Porsches are regularly rated as among the most reliable automobiles built.
2008 is right in the middle of the era when Audi won all of the awards for interior design (they don't do that as much now). Audis from that era are nicer inside than MBs and much nicer than old, stodgy BMW designs. Audi is a lighter vehicle than either of its German rivals - better gas mileage and more nimble handling in the corners. The A4 cabriolet was chosen by many automotive magazines as the most beautiful convertible of the era.
If you want to haul a lot of gear and go "for real" offroading, you want the SUV. For around town enjoyment in a beautifully designed car, you want the A4. You need to pick from these two based on what you want to use the car for.