The engine itself is great. However there is a somewhat problematic fuel pump on the 2011-2014 models. Mind you, I used to own a 2011, and enthusiastically educated myself all I could find about said problems. And, in the end, appears anywhere from 97% to 99% of the units sold since 2011 have been problem free.
Good news, except if you are one of the small percentage of owners that do have problems.
I understand one of the changes, besides additional horsepower to the 2015, was a redesign of the fuel pump.
So I would surely go with 2015 if possible.
Power wise mine was great! Hauled and towed my stuff just excellently. I was going to trade trucks this year anyway, as I normally do every four years or so. Then made the mistake of stopping at a dealership last fall, just to look you know, and ended up driving away in my new truck.
So now I drive a new Ram. But, should the urge hit me to trade again, I would not hesitate on a 2015 or newer Ford with the 6.7L diesel. I really think Ford has created a superb engine with this one, putting to rest the fiasco that the 6.0L and 6.4L Navistar engines created for them.
OH, and the 6.7L is a Ford built engine, not manufactured by Navistar for them.