A technician I respect a lot who does a lot of motor enhancing shall we say at a Ford dealer Said he would probably run an EcoBoost in a truck.
If you're towing it's low RPM turbo boost makes a huge difference but the 3.73 gears make it too snappy in many people's opinion off the line so unless you need Max tow get 355 gears. 5.0 get 3.73.
How you drive it as way more to do with the mileage than the motor..... around town in Cody Wyoming is much different than around town in Los Angeles. How you take off from stops and how much you idle can make a huge difference. No full size vehicle gets good mileage around town. Also the boost is addicting and it's easy to put your foot in it a lot. A Chev 5.3 would likely get better mileage.
I would get the 3.5 for resale unless it's just a driver the 2.7 should do a bit better on mpg's
I would be cautious buying used as turbochargers need maintenance and if oil hasn't been changed etc who knows how long it will last. Just like people who buy used diesels the tight tolerance motor and expensive turbos need their oil changed frequently and if they don't they wear and they're expensive.
Irregardless of tow ratings a half ton is only good for about 8,000 lb other than on-level ground. The trailer will push it all over the place due to the soft suspension and lack of brakes.
if you have heard of the class action lawsuit against the EcoBoost motors it is only for the smaller ones not the two seven and the three five.
in my recently purchased Transit work vehicle I chose the naturally aspirated 6-cylinder.... the 3-5 is an option.
my next work van will probably be all-wheel drive and I will probably get the EcoBoost.
Maintenance discipline ......and good luck!