Both, have to be able to take a trailer and have to be able to handle the weight of things in the box.
I have a friend hauling a large gooseneck trailer in the Ford F-150. They now have a second level towing package (max towing). They also offer a Heavy Duty Payload Package. You can get 11,000 lb towing and ~3,000 lb payload depending on the cab/engine/frame combinations.
He has been running his for a couple years I think, hauling that heavy trailer hundreds of miles nearly every other weekend chasing the rodeos. I plan on it for my next truck for the same reasons.
http://www.ford.com/resources/ford/general/pdf/towingguides/14FLRVTT_F150_Sep11.pdf
http://www.ford.com/trucks/f150/specifications/payload/