We run a small fleet of 6.7's in 4500 trucks (2008-2016). Like them for performance and really glad they have a 100k emissions warranty. The traps, catalysts ect. are very costly when they fail. We have several over 100k and hope all the problem parts are fixed now. So far so good.

The 6.4 closes the gap for torque and performance. Plus diesel is usually more costly per gallon.

In your application I would choose the gas engine personally.