First of all i tow a 13,000# 5th wheel and I have 3.31's. My truck has no problem with any hill or mountain I have ever encounted. I have been over just about every mountain range in the country. I have towed coast to coast twice both northern ans southern and across the middle. My truck never had any problem maintaining any speed. I never had to floor it to keep up. From California to Maine. From Texas to Michigan. and many roads in between.
With the 10 speed transmission gear ratio means nothing. The transmission will keep you in the gear that you need. I would however just go with the 3.55 unless you are going to hook up a plow you wont need 4.10's.
As for fuel millage. You are driving a truck that has 6 tires and punches a hole in the wind like a 747.
You like me didn't buy our truck for fuel millage but because it will do a job. Fuel millage never came into the equation.
Your truck wil get the millage it gets. Wind and load road conditions dictate fuel millage. That is why there is no fuel mill;age standards for Diesel trucks.
100% spot on. That 10 speed is a game changer. My 2018 F150 has the 10 speed and 3.55 with the EB, and effortless towing to GVWR and 12,500 GCVW. It pretty much negates the need for low gearing since final drive with higher gears is actually lower than the 6 speed with lower gears.