Diesel costs more up front and slightly more to maintain, but gets better fuel economy and has better resale. In terms of the total dollars you'll spend on either truck, money wise the difference is immaterial. Anybody who tells you different hasn't thought it through or is pushing an agenda. This assumes you'll keep the truck over a long period of at least 8-10 years.
In your case either will work for what you're planning on towing and where you're towing, so it's a matter of preference. If a 3/4 ton is significantly cheaper to register where you live I'd definitely buy one. The dirty little secret on RV.net is that most 3/4 tons are within a few hay bales of being able to carry the same load as a SRW 1 ton. The difference is almost entirely on paper versus real world, and your jurisdiction is a good example of why 3/4 tons exist.
Food for thought - I paid $39k for my truck in June of 2015, brand new. So I'd at least investigate new prices versus used to cover your bases, especially if you end up going with a diesel. Used diesels are crazy expensive these days.