Like others have stated, the gas engine HD trucks are all great quality. My friend had a 2015 with the 6.4 but he got the 3.73 rear end, not the 4.10's. He didn't like the big jump between 2nd and 3rd (I think it was those gears) when he was towing and climbing a hill. From what I understand the transmission ratios on the 6.4 are a little weird... but everyone that got 4.10 rears seems to love them. Never heard of anyone destroying the trans, but I've heard grumbles from people about the way they shift if you don't get the 4.10's.
My father in law has a 6.0 and tows about 8500 lbs with plenty of power and never a problem. That engine has been around for a while. Some people refer to it as a "tired platform" because its an older design... but I see it as a proven workhorse that has been able to stand the test of time.
I was researching Ford 6.2's when I stumbled across my current Cummins tow vehicle, which was used and traded in at the Ford dealer. From my research, the 6.2 is great engine and the transmission was very stout as well.
If I had too chose, it would be a combination between the best price and the most comfortable. I have ZERO loyalty to a brand, despite what a certain troll thinks. I've had a F150, Tundra and now a Ram 2500 and loved them all, but they fit different needs at different times in my life.