For what it's worth some real world experience.
Bought a 1999 F250 7.3L and towed my 28ft 5th wheel over 150,000 miles with 0 maintenance cost other than scheduled service. No break downs of any kind.
Traded for a 2006 F350 SRW 6.0L and towed 35ft 5th wheel 99,995 miles with no break downs or maintenance cost other than scheduled service. At 99,995 had a high pressure oil line break inside the engine. Fixed under warranty by first Ford dealer and drove 30 miles shut the engine off and it would not start. Towed to second Ford Dealer and they found that the first dealer left a piece of snap ring in the oil line and it got sucked through the oil pump, egr cooler, front engine cover and other parts. About 10,000 dollars worth of damage to engine, all fixed under warranty again. After that drove the truck for another 30,000 trouble free miles before trading for a 2010 F350.
Bought 2010 F350 DRW 6.4L and towed a 35ft and 43Ft 5th wheel for 32,000 trouble free miles but fuel mileage was awful so I traded for a 2013 F350 DRW 6.7L and have not towed with it yet but am seeing about 30% better fuel mileage running solo. Love the new 6.7 so far. I can't believe the difference in power from the 6.4.