If you go with the gas motor, the tow rating difference between the 3.73 & 4.10 gears is almost 4000lbs!!!! If we had found 2 trucks (bought off lot-dealer trade for current rebates) that had everything we wanted, but difference was the gears (3.73 vs 4.10), I myself would have chose the 4.10's for the slight advantage towing as we don't drive the truck daily.
And from what I have read. here and www.gm-truck.com, the mpg difference between the 3.73 & 4.10's about 1mpg from what people have reported when "empty". I think the bigger factor would be how heavy your right foot is!
IF you decide to go diesel, I would suggest maybe looking for a 3500 for the little extra payload capacity. If all else is equal (what I found looking for our truck with the 6.0 gas motor), I found the same optioned truck in a 3500 was actually CHEAPER by about $300 I think it was!!! Not huge, but still cheaper!!! Reason is it includes the tow mirrors, hitch pkg, and 1 or 2 other things, can't remember for sure.
Good luck deciding!!!
Chris