Just from another perspective.
Disclaimer, I own both 6.5's and powerstrokes, and cummins 5.9's, oh, and a 3116 cat, just in vehicles. Several smaller diesels as well.
The 6.5 in stock form, is a 6.5. No screaming hell for power, or at least not enough to hurt itself. That said, it starts every time, goes where I need it to go, and does what I need it to do. It has hauled hay, horses, cows, skidsteer loaders, small excavators, firewood, etc. It gets phenomonal fuel economy for being a 3500 4x4 dually, or even compared to a 1/2 ton gasser. They will get warm if you work them hard, and if you add injectors, or a chip, they are prone to cracking the webbing in the lower block, this is remedied by installing a main girdle. The older ones 92 -93, have mechanical pumps, very little to go wrong. The lift pumps need a relay, to deliver clean power to them, and that makes them nearly bulletproof. The later engines 94 up, have the DB2 electronic pump, with that pesky little PMD, or fuel solenoid driver, that gives nothing but grief. However, you can retrofit a mechanical pump on these engines with no problem. The oil cooler lines are prone to leaking, but they are an easy change. Dirt simple engines, for dirt simple people.
The powerstroke. Way more power in stock form, although pulling loads, I don't see a huge amount of difference, as the ford puts it down a little differently. More of a rev happy design. More fuel hungry than the GM by far. Overly complicated fuel delivery system, prone to failure at the most inopportune times. CPS is always an issue, carry a spare in the glovebox. High pressure oil o-rings have a tendency to let go right when you don't want them to. Turbo pedestal leaks at the EBPV actuator, not if, but when. Can lose a LOT of oil in a hurry. These engines tend to be a little cold blooded, if you don't keep them warm, many times they don't like to start. I have had endless starting problems with mine, to the point where if it doesn't start in two cycles of the GP system, I leave it parked and take something that will run.
Now, for a true "work" truck, I run a dodge with a cummins. Starts every time, decent on fuel, pulls like a locomotive, but it isn't a hotrod by any means. Just gets the job done right, and every time. As for the truck, well, dodge made the nicest packing crate I've ever seen, for an engine that should have been in a ford.... we'll leave it at that.
I don't see a problem with either the GM product, or the ford, for pulling something as small as a 5000lb trailer through the mountains. Test drive both, and go with what you are more comfortable in. That is really what matters.