FWIW on the mileage thing:
I had a succession of F350's with the "Tow Boss" option that was offered in the 200? -2006 timeframe (my last 350 was 2006). This resulted in (IIRC) 4.30 rear axle, 13,000GVWR and 26,000 GCVWR. My 5ver at the time was 16,000 GVW.
What I found was that with the setup I had, I averaged circa 11 mpg (like 10.8 or something) and that towing I averaged 9.9 -10.4. My BIL who towed a similar 5ver with a 350 with either the 4.1 or the 3.73's averaged circa 15 mpg non-towing but closer to 8 mpg towing.
If your primary use is towing, go with the gears that will make towing work...the economy gains are more imaginary than real with the bigger gears.