[JABURKHOLDER]

Torque gets you moving, horsepower keeps you moving. Rear end gears have nothing to do with the horsepower rating but has everything to do with how much torque is applied to get you moving. On an engine dyno, horsepower and torque are measured. Max torque is generated at a much lower rpm than horsepower. This is especially true in a diesel engine. Max torque is usually in the 1500 - 2200 rpm range. The rear end gears affect the rpm, the taller (higher number) the gear, the higher the rpm. The whole idea with the right rear end gears is to get you moving with the least amount of effort from the engine. A higher gear ratio does that by getting the engine rpm in the optimal torque range more quickly. The transmission controls all of that by picking the right gear to keep you in that optimal rpm range. The Tow/Haul mode on the Ford does exactly that. It keeps the engine at a slightly higher rpm, to maximize torque, for a longer period during shifts to get you moving. Ideally, you would research where your engine generates its max torque, find out the gear ratios in the transmission, do the math for the final gear ratio (transmission gear X rear end gear) and see what rpm ranges are generated. BUT, you're not building a race car. Almost everything in vehicles today is a compromise. Fuel economy, power, cost, etc.

All that being said, If I were in your shoes, I would go with the 3.55 gears. A very good compromise for your towing needs and daily driver.

Personally, my F350 has 4.30 gears and moves my the truck and RV (3791RD) (approx. 24500 lbs combined) at idle.

Just my long winded 2 cents.