If you tow your 8,000 lb trailer more than a couple of times a year, I would recommend going with the DuraMax. The transmission and engine are set up to handle the weight and you will be happier with it even though the mileage won't increase enough to offset the extra upfront cost in any appreciable way. We had a 35.5 ft goose neck travel trailer that we pulled with a Ford Powerstroke when we bought it, and later with a GMC DuraMax. We only took a couple of long trips every year, but made several round rips over to south Texas each year (usually about 1,600 miles round trip.) The DuraMax teamed with the Allison transmission was the far better performer. I had occasion to use my son-in-law's Chevy with the 6.0 liter gas engine for one trip, and I was literally grinding my teeth most of the time.
