I can appreciate the situation you are in. I think Porsche makes wonderful cars, except of course, for the price of entry. Please keep in mind that the Cayenne was co-developed with the VW Touareg, which itself is associated with reliability glitches, at least early on. So I would probably avoid any early Cayenne vehicle. Assuming that the Cayenne you're looking at is several years old, definitely get a pre-purchase inspection and make sure you investigate the weak spots of this vehicle if you do decide to go through with it. These vehicles have A LOT of sensors and electronic complexity.
Personally, if I was to get a Porsche, it would be one of their sports cars. The price of early 997 carreras is starting to come down to reasonable levels. I saw a 2005 (first model year) non-S carrera with like 30k miles for just under $40k. And of course, a mid-engined Cayman S is also a legit sports car (maybe more so than a 911!) and something else to consider.
In the end, I suppose it depends on how you are using the vehicles. If you need something dependable for daily driving duties, I would be hesitant to rely on a Cayenne, unless you have reason to believe that the example you are looking at shouldn't give you much trouble. If you can keep your f150 and get something fun, personally, I'd rather have a used cayman S/997 versus a mustang, but again, that's just personal preference. If the mustang is going to be a DD, I would be hesitant to get a 997/cayman S instead, however. Though I have heard good things about the reliability of modern porsche sports cars.