If I were to choose between Cayenne and X5, I'd chose the X5 4.6iS. The reason that I actually chose the BMW is because it's quick(just as the Cayenne), but the X5 has something the Cayenne lacks; exterior design. If someone of you people watched the Tp Gear episode where Jeremy tests the Cayennes 4x4 abilities, then you might've noticed that he ditched the car at the end because it was so boring exterior-wise. I quote Jeremy:
The Cayenne has a sex-appeal of a Camel 