What's the difference in horsepower on those two? I expect the 6.0 probably had the greater horsepower but you never know sometimes. Anyway if you are pulling a 28' trailer, there isn't such a thing as good gas mileage.
If you are going to be in hill country or mountains, I'd go with the larger engine. If mainly flatlands, the smaller engine.
I have to agree with Shadow. I have a ford F-150 with a 5.4 liter V-8. Saves a little on gas but bogs down substantially on mountain grades. Needs to rest on hot days to keep motor and transmission from over heating. So smaller engine more concerns not really that much fuel savings. My GMC 2500 8.1 liter (496) with automatic 5 speed Allison transmission has plenty of power and durability no need to rest pulling my 23 foot trailer. Gets about 12 miles per gallon on mountain grades compared to my f-150 that gets about 14 same driving conditions. In this case I think bigger is better.
I have pulled our Keystone 29rkssr with my 6.0 chevy 4x4 and pulled fine on flats and calm days. Two yrs ago we left on a trip driving into 25 plus mile winds averaged about 5 mpg going about 60mph and running high rpms. That was the a changing point we went to a 6.6 diesel. After pulling camper for 20 yrs with a gas engine I wish I would have switched sooner. Plenty of power better mileage per gallon.
If you can look past the expense, a diesel is the way to go. They are workhorses and have no problems towing unlike a truck with a gas engine. Even when not towing a diesel truck will get better fuel mileage.
The only drawback is the expense. They are more expensive to own, and repair, but you cannot argue their capability. As long as we have a trailer to tow, I will use only a diesel equipped truck. If you remember NOT to treat your diesel like it's a race truck, it should do fine.
I agree with artmart diesel will pull like a mule and like doing it.I went from a 1500 gas to a 2500 diesel pulling same tt no comparison. As for memberships we've never had any so can't help you their.