PoPo5.0: Gear ratio directly affects rated towing capacity. Hence why the F450 doesn't come with the lower gear ratio options. A truck with 3.55 would get better MPG on the highway while NOT towing, but the 3.73's might do better with a trailer in tow as the engine is not struggling as much. With these trucks MPG is more about the way you drive it than gear ratios.
On my truck, and I think most 6.4's, the sweet spot for the best MPG is just below 2000 rpm. So a truck with 3.55's would probably get the same mileage as a truck with 3.73's if the RPM's were the same on a given trial. As for going up in tire sizes, I'd think 3.73's would be ideal if you were to go with a 37" tire.
Lots of people on here have lots to say about this subject. But remember, math is math, and there's benefits to both ratios.