In February of this year I bought a left-over new 2013 Expedition with 4WD and the HD Tow Package. The 5.4 is an excellent truck engine with a flat torque curve. I graphed the torque curves of the 5.4 and the EcoBoost and they are remarkedly similar, except that the 5.4 has about 15% less torque across the rpm range. Same goes for horsepower.
BTW, I pulled the trailer on a 2300 mile trip this spring and averaged 11.5 mpg, hand calculated.
For my purposes (pulling a 3500 lb. travel trailer), the 5.4 is plenty adequate. For the size and weight of your anticipated trailer, I would definitely wait for the EcoBoost.
Bruce