I sold my '11 back in January after changing jobs made owning it a tad bit expensive.
From what I've read the 14-up are a little different than the '11 thru '13. Some issues have been addressed and I'd definitely go for the newer '14. One big change is to a larger turbo. This was a problem on the earlier "Scorpion" diesels, I never had a problem in the 65K I owned mine though, and it was an early 2011 job 1.
Overall I loved my F250 and wish I could have kept it. The actual engine never gave me a single problem, however the emissions system a radiator replacement sealed the decision on selling it.
There's a lot of emission equipment on these new trucks (all 3 manufacturers); DPF, SCR, EGR, DEF injection system and none are cheap to fix. FWIW also, Ford's emissions warranty is up at 40K, not 100K like the engine. This is all still deleteable, however the prices to do so have risen astronomically in the past few years.
They are great trucks that make huge amounts of power. Best of luck on your decision. I know if I had deeper pockets I'd have kept mine, although I do love the new T4R TE.
- Jeff