I have given this alot of thought as I have both vehicles. The duramax is an 02 extended cab long box 4x4 and the 6.0 is in my 07 crew cab short bed 2wd. I do some towing, my trailer is 26ft (just over 29ft total) 4700# empty
I've had the 6.0 a few years now, it's been a great truck. All I have had to put into it are tires, one wheel bearing/hub, and a battery. Yes, at least a couple exhaust studs have broken, not too concerned about it yet but it's somewhat annoying on a cold morning. The truck is consistently at about 13.5-14 mpg with about 80-85% highway driving. When I bought the truck it had half the miles anything else I looked at and was almost half the price. Duramaxes were still at least $5k more than that with 2-3x the mileage, or there were alot of extended cabs which don't work well with baby seats.
I got the duramax on trade for a snowplow I had on my 99 6.5 diesel. I agreed to take the trade with towing the trailer in mind and having a 4x4 again. Was a good deal I thought, but needed some work. This thing has lead a tough life but the injectors have been changed and it has a newer transfer case. Mostly the work it needed was replaced front fenders, cab corners, rockers, windshield, and needs a flatbed or box. Does need some mechanical work also, but not an overwhelming amount, most people around here turn down buying a GMT800 due to rust, of they just get scrapped/parted.
I've owned half a dozen diesels and as I get older, I realize that they just aren't for me. Living up here in the winter, a diesel sucks. They are always hard to start, always need to be plugged in, never heat up inside unless they are really working, they usually piss off the neighborhood, etc. Around here, fuel and reg gas are usually within 10 cents a liter of each other but diesel fuel usually has the upper hand. Once you factor in fuel treatment and antigel, gas is cheaper. Now, I like to travel in the States often, and fill up in Minnesota often as I'm 40mi from the border, diesel fuel is always more money than gas at least where I go in Minnesota, once you do the exchange rate diesel fuel is about the same cost in Canada and USA, if not more in the States. Gas is always less in the States, even after exchange. Right now it's about 40% less.
Now, I don't care too much about what the gas costs, and how much better the duramax is than the 6.0, do I really save much, do I really get much more MPG out of the 6.6? 95% of my miles are driving, 5-10% or so towing. 6.0 gets about 8mpg towing around here, it's mostly hilly. The thing that bothers me about the 6.0 is lack of power for towing. Granted I have 3.73s and am debating on 4.10 or 4.56, I've debated on a supercharger but I'm not in a hurry, I plan my acceleration based on the hill that needs climbing, etc. What bothers me most about the fuel consumption is not the 8mpg as much as I am in a pretty remote area, but we don't limit our travel to just around here. But around here, station are few and far between, and being in the middle of nowhere, they keep limited hours.
So the mileage is not great in the 6.0, but I don't know how much better the 6.6 is. Certainly not worth the headache, maintenance, repairs on the duramax IMO. Oil changes, filters, etc. I can hop in the 6.0, turn the key on a morning that's -45* and know that it will start for sure. Slowly cranking over, low oil pressure light on and certainly not good for it, I know it won't be leaving me pedaling go work.
Another thing I thought about that 6.0, I was going to buy a 2015 2500hd with the 6 speed. The internet is full of information stating that 6 spd is worlds better than my 4spd. I rented a 2015 2500hd and put 6000km on going to Colorado and back, wanted to check the thing out, see if it was really for me. Message center didn't move off 12.7 MPG and my hand calculated MPG from my receipts was better but not by much. We are also comparing a 2wd, 4 spd, 3.73 gears, shortbed with a 4wd, 6 spd, 4.10 gears, long bed.
I ended up buying a 5.3L, 4x4, 1500 crew with the 6.5' bed and 3.42s. Just couldn't get enough discount on the 2500. We'll see how the 5.3 does with the towing.
In conclusion, I'm still looking for a 2500 8.1L suburban/ Yukon XL.
