I love my 12, but everything that I have seen is that the 13+ is a better set up from MPG standpoint, upgraded towing & payload, and better interior.
The folks that I seee searching out the '10 - '12 CTD are the people that want to delete the emissions. That isn't somthing I am interested in.
As far as MPG, my truck is pretty much a dedicated tow vehicle. Truck has a little over 37,000 miles on it, and maybe 5000 of those are not towing. Lighest I tow is just my 16' fishing boat, heaviest is my 5er and ski boat tandem (17,000 pounds). Lifetime MPG on my truck is 10.91 MPG. Hand calculated.