I have a `13 F-150 with the 5.0 and I have been more than pleased with the performance the engine puts out so it shouldn't automatically be discounted in favor of the of the 3.5 Ecoboost. At the time I bought mine, there was a lot of talk on this website and others about concern in the turbos due to a design flaw in the charge air cooler causing the engines to go into limp mode.
I assume the problem has been corrected because I haven't seen it talked about in quite a while, so it's probably a non-issue with latter year models.
Like others have said, look at the payload capacity first because more than likely, you will run out of that before you run out of towing capacities.