I can only state that I have HEARD the 6.4 does not give the best mpg.
The 6.7 is a Ford in-house developed engine. The 6.4, like the preceding Ford diesels was developed by International. That's not a bad thing at all, but I think Ford got so much bad press because of the 6.0 that they decided to develop their own diesel engine - thus the scorpion 6.7 came into being.
I have a 2013 F350 6.7 and it is simply terrific. I never want to get out of the truck. It gets very good mpg and is super strong.
The 6.7 IS that much better. I cannot advise you about the pricing, but there is no question in my mind that I would want the 6.7.
* Ford does have an after-market warranty available for various periods of time. Maybe try and work a deal that includes some extra warranty time.
Tim