With the mountains you will find the Exhaust braking will be a tremendous advantage if you get a diesel.
The Canyon/Colorado with the "baby" duramax is a great truck and rated to 7600lbs has a built in brake controller, and exhaust brake. The diesels tow at a lower RPM and will make going up the hills much smoother as the truck will not have to downshift to hit the higher RPM of a gas engine.
Also if you drive the truck around much you will appreciate the real world 24-36MPG. (Yes i have one and have hit 36MPG with a 4x4 truck doing 65MPH for 40miles)