I'm new here, so let me start off by saying hi. I've had a Ram 2500 6.4L CC LB, with 4.10 gears since Dec '13. I use it to tow a 36' 9500 GVW Keystone Outback, I also live at about 5400 ft. So pretty much every time I tow it's above 5000 ft. The truck does a great job pulling the giant box behind it. There's plenty of power even with the thin air. Towing I can get anywhere from about 8mpg to 12mpg or so. Not towing is 14+.
I figured with my short city street drive to work & cold winters here I was better off with gas. I really like this truck, it rides good for a heavy duty truck, has almost 3k payload on it, 15,400lbs max towing & 22,400 GCWR. The diesel version would have only gained me about 2200lbs of max tow, but I would have lost payload with the heavier engine.
I personally think you can't go wrong with the 6.4 or the CTD, I guess it just depends on personal preference & how often you tow. The way I looked at it when I bought the truck, is the CTD was about $6500 more than the 6.4 & since it is the same engine used in the SRT8 & Hellcat Challengers just retuned, I could always spend some of my savings & buy a blower for it if I needed more power. :B
The only thing I would change if I was to do it over, is, I would have bought the Ram 3500 to pick up about 1k of payload.