Get the 3.55. Not towing you may not get better mileage, but towing it is just right. The F250 with Diesel (4x4 I have with shell) tows fine and has excess power going UP, when needed in the mountains and loss of engine efficiency power, due to Elevation.
I tow with a 2016 F350, 3.55, Diesel and tows our 27 foot loaded comfortably and anywhere. Mileage will run 10 to 12 mpg on the Highway and 8 to 9 up mountains, less depending on which mountains. You pay going up in mpg. You receive mpg on the way down.
Braking is a dance. The Diesel engine will brake, but towing... people are dreaming if the compression will keep you from accelerating on a Colorado Mountain Pass without laying onto the brakes in short period to maintain control.
We have towed a 25 and a 27 foot with the Diesel. Excellent. Over 50,000 miles towing. I would have to check our log book for exact... but no transmission, brake, engine or cooling problems. No warranty work. No repairs. The front wobble is expected on the 4x4 and the computer screen goes in and out at will... and no Ford Fix as they have no chips.
Flat land towing. Easy. Mountains... out west, you learn by trial and error. Our RPM is often in the 1300 to 1500
RPM going UP. Going DOWN our Brakes trump the Diesel Braking as it can climb in RPM's as you think is working.
We have the original brakes. Do not smoke them. It is a dance of Engine and Brakes in the mountains. I would have to be riding with someone to get an idea of how they do the... DANCE. Some... you could not pay me to be a passenger.
Smell HOT Brakes in front of you? Get control of your F250 quickly... stuff begins to happen. This is not something you will learn in a manual or short posts. It is experience... learning by doing.
Lighter vehicles... mountain towing? Go for it. Not us. Been there, know the difference in vehicles. I enjoy driving the F350 for picking up doughnuts and fritters in town... without worry. A tank in town and a wonderful choice towing in the mountains of Kansas or Colorado.
You may get a lot of mountain towing advice from Flat Landers... listen to the High Country towers. They do it all the time. Utah, Colorado, New Mexico and Wyoming drivers watch the License Plates... closely.
For good reasons, too. Smell their brakes and watch the tail lights... on the way down. Going up... is for everyone. Running engines/transmissions HOT or Warm is a choice in vehicles... not weather.