If you are coming anywhere near the manufacturer's "claimed" payload or tow limits yes, I would step up to the next level. However, I am towing a 4500 lb Airstream with a F150, HD payload and towing package, with a claimed 9,000 lb towing capacity and I barely feel the TT back there. I can also floor it and knock things off the shelves with the acceleration. Some say that's fine but, the 150's and 1500's don't have the brakes to match the claimed payload and tow capacity. I stop fine, even on medium grades and, in steeper terrain, in tow/haul mode, the transmission is amazing at knowing when to downshift to help slow you down if you start rolling downhill too fast. It is kind of like a fake jake brake.
Again, if you stay reasonably UNDER the claimed limits I would go for the new ½ tons. It sure pays off when you disconnect and get 20-21 mpg with the Ecoboost etc. BUT, if I was going to push the limits now or possibly sometime in the future or, if was using the truck almost exclusively for towing and wouldn't benefit from the disconnected high mileage of the ½ ton, I would have moved up a notch also.