As it happens, I have a GM 5.3 and an Ecoboost. Yeah, my 2010 isn't an 'Ecotech,' but I can't imagine the fuel economy improvements to the motor make any difference when towing.
Anyway, they're easily w/in 10% of each other towing. The 5.3 'seems' to be working a lot harder, ie, higher rpms and all of that, but the end result is about the same as far as mpg goes.
I prefer the towing performance of the EB over the 5.3 by a good margin, but fuel economy is not the reason!
FWIW, the EB does seem to get better mpg for daily driver/commuter use -- my average commuting tank is around 18.5 in the EB, where is was closer to 15 mpg when I was driving a 5.3 every day.
Good luck!