You guys should pay more attention to the OP's handle (name). Her fiance is the one with the big truck.
I'm in the process of shopping for a new-to-me (used) F150 as well. I've been looking at the towing charts. It looks to me like the 3.5L eco-boost is the one rated to tow heavier loads than the 5.0L non-turbo. As others have said, you must have the max tow package to get all the way up to the top towing numbers.
I've owned a 5.0L non turbo for over a year, and have used it for towing a much lighter trailer (4500 lb or so). I test drove a max-tow 3.5L eco boost yesterday, without a trailer attached. I was amazed at how much more acceleration it has than the 5.0L. It looks to me like the eco boost is a beast.
fwiw
-Speak