Before buying anything I hope you will read this article at Truck Camper Magazine http://www.truckcampermagazine.com/news/tcm-debuts-2014-ram-3500
I think they make an excellent argument for buying the Ram 3500 with the 6.4 gasser the beauty of this engine is that four of the cilyndrs shut down when not needed. this feature boosts the gas milage to diesel levels without all the additional diesel costs.
I own a Ford F350 Dually with the 6.4 diesel. I love the truck, but I don't think the extra money is worth the little additional gas milage that I get. frankly I think it is more truck than I need.