Perhaps you should just go test drive both? I don't see many people making the same jump as you.
I have no experience with any S-Class so I can't comment. However, I can speak about the new Cayenne; my friend(Porsche Sales) recently brought over a Cayenne S for my father to try. Coming from an X5, he was very happy with the way it drove and felt. The quality was leaps and bounds ahead of the X5. This generation Cayenne doesn't drive like an SUV, it felt like a car; it almost felt as planted as my M3 on stock suspension. Needless to say, he put an order down for this car to replace the X5. Although, I should add --- no matter how great this car felt, there's no way I would want to drive something like this unless my body is incapable of handling a sports car. I really don't see a reason why anyone under 40 would want this as their main car and only car.
By the way, have you looked at the waitlist on these cars? The V6 model is very popular, order now, pick up June '12. The V8 model is a tad better, order now, car arrives early Feb.