Definently go with the diesel, gassers are simply weak in comparison, less efficient, less power, etc. As many others have already said, you'll be fine with single rear wheels. I'd go with the 3500, they typically have overload springs stock (some 3/4tons don't) and sometimes a higher GVWR. There are tons of used diesels for sale on the eastern slope, there are a couple lots just off I-25 just north of Denver with good selections. If you tow multpile vehicles a lot be sure too pay attention to GVWR's (gross vehicle weight ratings), the higher the better. The biggest short coming of my Dodge 2500 Cummins is the lowish 9000lb GVWR, the truck weighs 7000lbs+, that only allows 2000lbs tounge weight(gooseneck). Other than the GVWR, I'm very impressed with the diesel trucks, I recently towed a 11500lb load/18000lb+ Gross (gooseneck) across country cruising at the speed limit with no problems at all. Check out
www.dieselplace.com for a forum dedicated to the diesel GM's.