I've got a 2000 3/4 ton suburban with 6.0 engine and 4.10 rear.
Best non-towing was 12-13 mpg
Towing my Formula 303= <8mpg.
You never said what you're towing?
If it's over 6000 lbs. go for the diesel, if under I would just get a 1/2 ton with the 5.3 engine. I heard they get around 20-22 mpg highway. That was from a friend with his tahoe 5.3
Bottom line is for heavy towing, the duramax/alison combo works the best.
And diesel is about .35 cheaper than gas now.