I would think the F350 would perform better and the even with the hgiher price of diesel, the improved fuel economy of the diesel in heavy load towing conditions might be more economical. Not losing engine performance at high altitude would really help in Colorado. I do have to ask, why have an 08 (new) F350 diesel if you're not going to use it for towing.
From my experience, the Expdition is great for towing. In 1 year I've towed 4,000 miles with a 31' Trail Cruiser 30QBSS ultra lite travel trailer weighing #4,400 loaded and 7-8 passengers (GCVWR #13,000-#13,500). MPG usually averages out to 9.9MPG.