For towing, I pretty much dig the 4.10 with a four speed auto, though 3.73 with an older 3 speed auto in a truck works well and is just about perfect with a four speed auto on a V8 powered car.
The reason I would go with the 4.10 has to do with the final drive ratio which must include the tire size. Trucks use big tires. When you compare the actual distance the wheel travels per revolution of the drive-shaft, between a 3.73 in a car and a 4.10 in a truck (with car versus truck tires) you're closer than you think.
This is a neet little comparison chart. You can put in your calcuations and compare them side by side. Take something you KNOW and compare it to the vehicles you are considering to get an idea of the RPM you will be turning. Also, remember to consider the power band of the engine you are buying. A diesel makes makes tons of low end torque and can often take a taller rear gearing where with the gas engines, maybe a little steeper (toward the 4.10) will work better to put the engine in the power band.
Gear Ratio Calculator
Also, the huge killer when you are towing a trailer is wind drag. Tall trailers really kill the mileage, especially when you pick up even a few MPH. Keep the speed down and you will really improve your mileage with a tall trailer.
angela