Diesel costs more to repair, costs more per gallon, costs more to buy, and I own one for a reason...
#2 Diesel has 14% more BTU's than unleaded, so you will not see a 50% increase in fuel economy. At 18-19K lbs GCW with my sig truck and trailer I get 9.5-11 at 65 depending on the winds and elevation change. The gas rigs I go camping with get about 7-8 at 5-7K lbs less weight, so there is a difference there. Cost per mile, not as much as mpg.
Diesel economy also takes a hit because now you can pull grades at 65+, where before you where at 50. Run the same speeds as before and the fuel savings will be much larger... but I doubt 1% of people do that.
I would steer as far as possible from a 6.0L Ford, or even the 6.4L.
If you have to go 8-10 years old you are going to want Dodge or GM for reliability, Ford just doesn't have it.
Anything 07.5 and newer will get lower fuel economy per hp than older trucks. Some of the newer DEF trucks are better than 07.6-10 but you have to add in the cost of DEF too.
Don't dwell too much on the displacement of the diesel engine, look at the power. A turbo does wonderful things with smaller displacement.