My dad's 2005 duramax never did as well as I thought it should. Lucky to get 18 on the highway and it got about 12 towing a small-ish travel trailer. The 6.0 gets 12-14 empty and under 10 towing from what I've heard. Buddy had one in a 1500HD with 4x4 and got about 13 mpg empty.
Basically, they both drink it. I'm a chevy guy too, but if you want a chevy that can tow more than a few thousand pounds, they don't have anything that's fuel-efficient or reasonably priced (much less both).
Life can only be understood backwards; but it must be lived forwards.
- Soren Kierkegaard