The heavy gas trucks do get terrible mileage, but that's offset at least some by the cheaper oil changes and the fuel filters you have to do on diesels. And the 6.4 supposedly does a little better unloaded because it has cylinder deactivation. Still, you're definitely going to be paying a bit more for fuel.
A more practical difference between the two might by towing range - with the gas truck, you're going to need to be more careful about planning for fuel stops and you'll be stopping more often. You live in a more populated state, but if you plan on traveling out west that becomes a bigger factor. Just something else to think about.
Edit - go drive both too. Some just flat prefer gas or diesel. On the Ram, the 6.4 and CTD feel like very different trucks from the driver's seat.
