If you're buying a new truck then I'd say all 3 makers are doing pretty well right now. If your talking about a used truck the stay away from Ford 6.0 and 6.4 diesels. Also stay away from the Chevy Duramax in '01 thru '04.5. These years it was the LB7 and had lots of injector problems. Dodge trucks (gas and diesel) up until the introduction of the 68RFE had lots of automatic transmission problems. I would go with a manual unless it's new enough to have the 68RFE or now the new Aisin.
Diesels should get better mileage. The newer ones don't do as well as the older ones. (Thanks EPA).
My '01 Chevy 4x4 dually had an 8.1L gas engine and got about 10 mpg.
My '05 Chevy 4x4 dually has a Duramax diesel engine and gets about 16 mpg.