You got a tough one on your hands. I did drive a 2011 5.0L F150 company truck for a short period of time(about 6 months) before I got a newer 2012 F150 Ecoboost. I will say that it did not feel as strong of a puller towing the same trailer as the 2012 especially at low rpms and got worse fuel economy. Since the Ecoboost is out, I would say that the Ram probably has the more powerful engine with better fuel economy if it has MDS, but the Ford has the better suspension and all round set up for towing especially if it is the HD or max trailer tow package.
Good luck. The only way I would go back to towing with a half ton after owning a diesel is if it were another Ecoboost.