Ask Mr. Truck #8: Cummins or Duramax? Ram or GMC 3/4 Ton?

Which truck to get: a 2017 Ram HD 2500 with the Cummins or the 2017 GMC Sierra HD 2500 with the new Duramax diesel? Is waiting for the 2018 Ram HD a good idea? Will the first year of the new truck have reliability problems? Also, fuel additives, gooseneck vs. 5th wheel, and more.