" I know very little about the mechanics of trucks and rely on weights etc. I do know that the 3.73 is less favorable than a 4.1 ratio for towing - but how much?"
Just about exactly like the towing numbers show. Chassis - wise, F250s and F350s are very similar, almost the same and save for rear springs, have relatively the same capability across the board.
Drivetrains are a different story. The 2 - 6.2 3.73, 6 speed F250s I had got 7-8 mpg towing about anything over about 5000lbs and they were all low profile loads (flatbeds with materials or equipment on them). Empty driving mileage around 13mpg. Bulletproof setups but lacking in power. 2nd and 3rd gear pulls up mountain grades with anything substantial hooked to the back.
Most I pulled was 12-14klbs, flatbed with street plates and it would run at normal speeds no probelm, but there was zero wind resistance. Just towing a 5000lb cargo trailer was a similar feat as the heavy trailer sat highway speeds.
If you are only towing the 5ver occasionally, then it is ok, IMO. If you're going tp be using it regularly for towing, it'll drink gas like Charlie Sheen and be sucking wind on every grade.
There's a reason the price is attractive. It's because the performance is not. It literally has less than half the torque of the diesel.