In my view the rear axle ratio is only a small part of the equation when it comes to MPG. You also have to take into the equation how you are going to be using the truck. Is it going to be a family grocery getting vehicle, is it going to be used as truck carrying a load, towing a load, city driving, highway driving and how heavy is your right foot.
If your going to be running pretty much no load on the hiway a 3:15 would net you the better MPG
If your running say a 50/50 mix hiway to city empty to lighter load, I would go 3:55
If running city more than hiway, or carrying/towing a heavier load I would choose 3:73 (which I did)
As for me with the 3:73 ratio I don't watch the MPG, I drive it as I feel in the mood to drive. I like the off the line in town TQ that the 3:73 offers me, I like knowing that the 3:73 gives me the ratio I need to pull a trailer and not suffer lack of power.
Also keep in mind what you may have planned for any future changes in tire size, my son had an Eco FX4 with the 3:55, then added 35" tires along with 4:10 gears which kept things close to where they were to start in overall ratio.