I think the presumption is the OP doesn't have an inherent love of diesel so as long as the gas engine functions reasonably well, it comes down to cost....otherwise why would the OP ask the question...
So it's logical that he's looking for objective criteria which basically boils down to can the truck reasonably pull the trailer and how much does it cost.
I'm the OP....I'm actually a NUT about Diesels.....Had a Mercedes, an Isuzu and a Diamond Reo.....I "GET" the whole Diesel thing better than most. Before I bought my 2006 Dodge Diesel I had a number of gas tow vehicles. They all sucked compared to my diesel.
But "new" diesels are much more complicated and expensive AND the modern gas engines are getting real close in performance and are considerably less expensive .......looking at Dodge Tradesman versions the cost difference is 33% ($33K for gas VS $43K for exact same truck with diesel)
If others that are towing a 6000-8000 pound trailer with a 6.4L and are happy with the combination, that's all I need to know. I trust the opinions of the folks on this forum more than I trust the truck salesman at my dealer.