I did a lot of research, talking to actual owners and spent a lot of time looking at you-tube reviews and came to the same conclusion as the guy in the link bsa provided. While the 3.5 can be set up to tow more weight, both are limited by 5,000 lb rated hitches from the factory. And I think you'll run out of combined carrying capacity before you get to the max trailer weight.
They found a way to get 25 more HP from the 5.0 in the 15's and 16's and the aluminum bodies are about 700 lbs lighter than my truck. A 15 or 16 set up just like mine is rated for 9,000 lbs. Mine is rated at 7700 lbs and more than I'll ever pull.
From what I have determined the only real difference is that on hills the 3.5 wants to run about 2500 rpm's and will stay in 4th or 5th gear. The 5.0 will maintain the same speeds, but wants to run closer to 4000 rpm in 3rd or 4th gear. You'll get to the top either way.
The 3.5 is supposed to be able to tow around 12,000 lbs if set up right. But I think you are getting into 3/4 ton truck territory with that much weight.
The 5.0 is using proven technology. I feel more confident that it'll be trouble free for 200,000 miles. The 3.5 might be just as reliable. But the technology isn't proven yet.
