I drove both quite a bit during my few months at Venturi. Fully-laden Chevy 3500's (1-ton). The older vans had the 6.0, the newer ones had the 4.8.
The 6.0 was a real hot rod compared to the 4.8, especially off the line. The greater torque of the 6.0 got up to speed with less drama and didn't downshift as much during acceleration or going up grades. I doubt there was a huge difference in fuel mileage but I can't back that statement up with facts.
The 4.8 wasn't terrible to drive by any means, but if I were to buy one and load it up with TM, water, tools, etc. it would be the 6.0, hands down.