I wrestled with this same decision for quite some time. I finally opted to go with the diesel 2500 without the mega cab (which is a big hit in towing capacity). My truck is rated for 18K+ towing a similar weight package to yours. Tows easily with excellent performance. Without a load, I'm pulling about 20-22 mpg on the highway. Don't think you'll see that mileage with the Hemi 6.4.