If you are towing a lot at altitude, the advantages of the turbo Ecoboost will outweigh the benefits of the V8. There are posts on the internet which show the % drop in power at altitude on naturally aspirated engines vs forced induction (turbo) engines. The 5.0 litre has a beautiful rumble to it and there are quite a few complaints about the gas mileage of the Eco, but complaints about the Eco's low end torque, particularly in the mountains, are extremely rare.
As stated in other threads, the 10,200 tow capacity that you refer to is really artificial as you will run out of payload, with a proper tongue weight, long before you get any where close to the stated towing capacity. On the F150's the payload capacities vary greatly depending on packages and options.
2015 Eagle 284BHBE
2010 Ford F150 XLT 5.4 Maxtow, Coverking Seat Covers, Putco LED interiors, Kenwood DNX571TR GPS, Kicker Powerstage, Weathertech Floor Mats and Window Vents, Line-X, Firestone Airbags, Michelin LTX-MS2, LivewireTS 5 Star tuned, Propride 3P