For starters, I'm assuming from your post that you're interested in the diesel. I've spent a lot of time in the build and price section of the Ford web site trying to figure out all the options on the 2017 F250 diesel while spec-ing out my own truck order. I know the gas models offer completely different rear end ratios.
Anyway, the 3.31 rear axle comes standard with the 2017 F250/F350 SD 6.7L diesel and the base 18" wheels. You can choose the 3.55 axle with this configuration but you will be required to get the Heavy Service front suspension package as well.
Also, if you select one of the 20" upgraded wheel options you will only be able to get the 3.55 rear axle. This is because the 20" tire and wheel combination is taller so the shorter 3.55 gearing is required to maintain the same overall ratio as the 18"/3.31 combination. Ford has apparently determined that this is the best gearing setup for general use with the diesel.
The 18"/3.55 combination is the odd man out in this scenario. The shorter overall gearing is designed for heavy towing and payloads which is why it's paired with the uprated front suspension. Engine revs will be higher at any given speed in top gear with this setup.
Since you say that better economy is your main concern, you should therefore go with the tallest (lowest numerical) gearing you can get. As you point out, the diesel has so much torque that it won't be an issue towing a TT anyway. This means that if you are ordering a truck with 18" wheels, get the 3.31. If you order 20" wheels you won't have a choice; you'll get the 3.55 automatically. BTW, I suggest you order the electronic locking differential option no matter what numerical ratio you choose.
I hope this helps.
“The thing about science is that it's true whether or not you believe in it.”
― Neil deGrasse Tyson
2018 30' Classic Twin (sold)
2020 25' Globetrotter Twin (here!)
2017 F250 4x4 6.7 max tow Platinum