We have 6.0's and 6.4's in all of our work trucks, F450's and F550's. Most if not all of the 6.4's are on their second engine. These trucks are worked much harder than anyone's personal vehicle.
I would never ever look at a 6.4, but I think I'd consider the 6.0 if completely bulletproofed. I think the tunes and upgrades is what kill most. I've ran mine low on coolant, waaay over for oil changes, and drive the h**l out of it, and it's somewhere around 14K lbs.
I'm on my 4th Chevy and have never owned anything else so take it FWIW !!