For a luxury SUV, I love the Lexus RX. May not be as flashy as the Porsche, but it's so plush. Out of the two you mentioned, I would go for the Porsche. I know it's like sacreligious on this forum, but I really don't like BMWs. They just don't have that cushy You should also look into the Mercedes GL class. Mom mom was going to trade in her RX for one last week, but fell in love with the CLS 55 so randomly got that instead!
Although I love BMW, I don't like the style of the X5 that much. And I have always loved the Porsche Cayenne, it's a beautiful car, I would definitely get the Porsche if I were your mom. Let us know what she decides.
I love SUVs and I used to have a Grand Cherokee myself which was an awesome vehicle... but I just can't stomach the cost of gas these days for them! So my vote would be the car that gets the best gas mileage, which is most likely the BMW (although don't get me wrong, I love Porsche cars, but in order for the Cayenne to have performance that is close to their smaller cars, I'm sure the gas mileage is abysmal).
Recently had this debate with my husband about which he wanted, and he's pretty sure he likes the BMW better. Even though the Cayenne IS a Porsche, it's not quite as cushy as the X5, and unless you get the S version or higher, the pickup crappy, especially considering its family tree.