In my experience, the new diesels with DEF are much better on fuel than the ones with the 'afterburner' exhaust.
I drive other people's trucks...no payments..haha.
Coincidentally, I just put 300 miles on a brand spankin' new F-350 today. Averaged 13.1 MPG pulling a loaded trailer. The truck it replaced was an '09 F-350 with a 6.4 and the afterburner exhaust; on the same run it would get 7-8 MPG.