The best load carrying tire for the single rear wheels are the 18 inch. Duallies come with 17 inch. Right there you're ahead by $400.00, but you need two extra. I had a 2500HD Chev 2011 Duramax pulling combined wt of 23,500 lbs and got 10.3 MPG. I now have a 2014 GMC 3500 Dually pulling combined wt of 29,100 lbs and get 10.3 MPG The major pain in the Dually is dropping the air pressure on the rear tires back to 40 PSI when not hauling and airing up to haul. My other observation was hauling in the mountains it runs one gear lower but no effect on mileage. The other observation was the electric fan kicks in periodically with the heavier load. I've only put 10,000 miles on the Dually but I think is slightly better on fuel but if it is not enough to take to the bank.
The down side is awkward to park, larger to wash, less liked by most women drivers. Both have done an awesome job. The 18 inch tire were not available for all companies like the 17 inch and that might be with the heavy weight rating.