At that load I would want a diesel for the power to accelerate onto a freeway and to have the engine braking going down a steep mountain road. With a gas engine I would go with the biggest I could get and get 4.10 gears.
With the diesel trucks the GM in the 2011 year have had the best overall performance when towing and the strongest frame but with the 2013 model year the Ram trucks improved the frames on the 1-ton trucks and Cummins improved the exhaust brake performance a good deal.
When I was looking for a diesel in 2011 I found that the asking prices for used trucks with less than 60K miles was at $35,000 and up. I ended up buying a new diesel for $42,000 instead. Used Ford diesel sell for a lot less than the GM and Ram and I suspect that is due to their poor reputation from the prior generation of Powerstroke engines (which no longer applies).