Tow vehicle and everyday driveability question

I would prefer the 5.0 to the Ecoboost in the mountains for the extra engine braking capability. Coming down the mountain should be your concern, not going up. The F250 will do better coming down the mountain than either of those two, but your current truck/trailer combination should handle it fine. Maybe you need a better sway control system like the Equal-I-zer 4 point. You never really said what concerns you about Colorado.