I know MPG is a consideration with today's prices but I'm guessing if you compared 3500s from all 3 manufacturers the fuel mileage will be very close.
To me comfort and build quality plus performance both going and stopping are the most important to consider. I had a chance 2 years ago to test drive over the same test track fully loaded 3500s from Ford, Chevy and Dodge. I found the Chevy to be most comfortable with the best ride and build quality. The Ford had the most power and I would place the Dodge 3rd. The Dodge had the worst ride and felt sluggish.