Because trucks are about torque, and I tow with my Expy, I'd go with the 4.4 diesel V8.
And IMHO, once you have that, they react much better to chips and custom tunes than gasoline engines, so even at a premium option, if available, between the better mileage, longevity, torque and power potential from a tune, more bang for the buck.
What's not on the original list, and really shocked me, would be the new Ecoboost V6 that's in the F150. For me, nothing comes close to the feeling of a V8. But if you check this out,
Ford F-150 EcoBoost Torture Test: Logging In Oregon - YouTube, this thing makes 425 FT LBS of torque right out of the box!
Now I would have to spend $7000 for the Whipple supercharger to get 450 FT LBS. I'll bet it won't get the mileage of the V6. At the risk of loosing that V8 rumble, that new Ecoboost might be a worthy option
and cheaper than the 4.4 diesel if they would ever offer that.