Just answering your question, for that load, I'd definitely go with the 3.5L Eco engine.
Now I will echo many others who have offered advice. There is no way in hell I would ever think about towing the trailer you plan on towing with even an HDPP F150 or GM/Ram equivalent.
IMO, that size trailer screams for a 3/4 ton or bigger truck. Currently, I tow a 27', 6500# GVWR trailer with my 1/2 F150 Eco with HDPP. I feel that combo works great. However, there is no way I'd hitch and pull the size trailer you now have to my F150.
I have priced out as comparably equipped as possible F150 and F250 gasser, both in XLT trim. The price is within 2 or 3 hundred dollars of each other.
My advice is to get a 3/4 ton gasser, or if you want diesel, get a 1 ton model.