My '17 F350 has the 3.31, and I have no complaints. I tow a toy hauler that weighs around #8000 loaded, and I seldom see 5th gear, usually I just hum along in 6th. According to the truck computer I get about 19 mpg in all around mixed driving, and about 12 towing.I can get 21'ish cruising empty on the freeway.
I towed my current TH with a '14 Ecoboost and Max-Tow (3.73) and it pulled well, but the cargo capacity was always at or over the limits. I had #17xx for cargo capacity. The Eco got around 17.5 mpg all around, and 9 towing.