You would need the 3.55 axle
You would need the 3.55 axle to ensure that you have comfortable towing even when using your Tow/Haul mode. Now I say this becaue anything over 5000 Lbs. Is getting heavy Into heavier hauling. Moreover the ecoboost verses the 6.2 is a negligible difference especially when it comes to MPG whilst towing. However, your overall MPG is going to be better with the ecoboost especially in the half ton market, especially for a daily driver.