CON: A Cayenne with a V6 is NOT a Porsche. It's a Touareg with a stupid dashboard (especially radio controls).
I don't know about better reliability than the Jeep. I would stay away from the MB, personally, for a multitude of reasons. I think your top two choices should be GC / Cayenne, with the Cayenne being better overall. They do handle pretty darn well, just too slow for the money. But, if you're looking at fuel economy, the faster you want a Cayenne to go, the worse it gets -- MUCH worse.
If you find that comfort is a selling point, the Cayenne is better all the way around, but you're going to lose some storage, if that's important.
You will have a bigger bill at the Porsche dealer when you go to do maintenance on the vehicle. However, if I remember correctly, it's a one-a-year maintenance, depending on how many miles you drive.
I'm sure that I didn't help much. But, overall, the Cayenne is the better SUV.