I have had 2 Dodge quad cab 1500's , I now have a Dodge crew cab 2500.
Those have been gas and work very well, the 2500 pulls trailers MUCH better! not even a comparison so if in doubt go big here. They make single wheel 3500's and the difference is more than suspension and breaks, heaver = lower gas mileage no matter what engine.
The Crew cab has the most leg room and you can take a trip with full sized people, no problems. The quad cab can get cramped, all the manufacturers have crew cabs. I would suggest the crew hands down.
I have had 2 wheel drive trucks and I will not buy anything but 4 wheel drive. My reasons are hunting, boating and winter snow. It is awesome when you need it, and you will need it.
It depends on how much and how often you will pull your trailer. Many gas engines will do fine but eat gas. On the other side if it is infrequent the trade off might be worth sticking to gas.
Happy hunting and pictures