The RX 330 is rated to tow 3500lb but I wouldn't. It is a very comfortable, light weight, stone dead reliable, well mannered grocery-getter/dog-hauler and travel vehicle that gets 22-28mpg. My wife is on her second one and it is an excellent compliment to the Boxster. Good choice 986.
The Cayenne S is a fully capable off road vehicle with a heavy duty chassis (1200lbs more than the RX), gets 14-18mpg, is rated to tow 7700 but I wouldn't. It is a lot more chassis but still a relatively short wheelbase so I would not tow more than 5000 lbs. We have several in our club that tow race cars with a Cayenne to good effect but all up weight is under 4500 lbs.
These are two different purposed vehicles with the RX being more civilized around town with 50% better gas mileage. The Cayenne is more all weather, all purpose, off-road capable but hauls around an extra 1200 lbs in chassis weight so gas mileage suffers.
Choose a truck chassis for serious towing of 5000 lbs or more and you will be glad you did.