OP says wrote:
But, it looks like the ton / 3/4 ton on the 3.73 has the same towing capacaties, is that right? If so, what's the benefit of a 2013 ton Ford GAS over a 3/4 ton with the 3.73 axle ratio?
Its all about payload differences in the two trucks.
looking at Ford Truck Body Builders advisory web shows the F250/F350 SRW with the 3.73 axle ratio are within 100 lbs of the same tow rating 11900 up to 12800).
The F350 SRW has up to 11300 GVWR with 7000 RAWR and a 4980 lbs payload.
The F250 has a 10000 GVWR with 6100 RAWR and payloads up to 3700-3800 lbs. All depending on vehicle selection.