Well, I would eliminate the Ecoboost. It's a great engine with gobs of low end torque, lot more than both those V8's, but when you use that torque you burn gas.
So if you're running up and down hills on cruise control with a 5000# trailer, the Ecoboost will behave as if the trailer almost isn't there, except when you have to fill up the tank.
For a 5000# trailer, one of those two V8s would have enough torque and probably give you better mileage towing. But I'd probably buy the Ecoboost anyway ... :)