I'm averaging 17mpg in my Tundra, so there's that.
I think Fuelly is good for comparisons, but everyone drives differently and uses their vehicle differently.
Driving style plays a huge role in mpg's (it accounts for as much as a 30% variance in mpg results according to one study). So too does climate, terrain/topography, traffic patterns ect. So someone driving flat, open highway commutes at 60mph is going to get a different combined average than someone driving up and down mountain passes in the dead of winter.
The point of a website like fuelly isn't that it will accurately predict what every single driver of truck X will get for mpg results, but rather that it provides a good baseline for efficiency trends with different vehicles, and different configurations of those vehicles.
The 16 mpg that a 3.5l ecoboost offers certainly is better than the Tundra's 14 mpg average, but also does prove that these ecoboost engines don't necessarily live up to the EPA ratings (~19mpg combined). As well a 2 mpg difference isn't huge IMHO...you're going to be paying a lot for fuel with either vehicle.