There is a much bigger difference in fuel mileage dependent on driving style than there is between any of those engines you listed. In other words, they're all so similar is isn't going to matter.
If you talk to someone who drives with one pedal or the other to the floor like some people do, they're going to get terrible mpg. Another person can take the same truck and get 30% better mpg by not driving it like a caveman.
There are plenty of comparison tests online you can read that show the mpg between those engines (from the 3.5L EcoBoost to the GM 6.2L) are very similar. In fact, I recall one test where the GM 6.2L got better mpg in the same conditions than the 3.5L EcoBoost. It's either Eco or Boost with that one.
Pick the one you like best.