MPG for 3/4 Yukon XL? How much worse than 1/2 ton with 5.3 liter engine.

I would check out www.fueleconomy.gov, they will have the gas mileage ratings. Everything I have read indicates that the trade-off in ride is typical between a 1/2 ton and 3/4 ton vehicle -- the 3/4 ton will not ride as well unless it is under a load, since it is designed to ride correctly with the load being hauled, and will be less comfortable without a load.

the mileage won't be as different as you think, because the 3/4 ton doesn't have to work as hard to tow it. 16-18mpg empty on the 3/4- 20-21 on the 1/2 ton, but that will drop alot towing because the 1/2 ton won't shut down the 4 cylinders the way it does when you are just driving (to conserve fuel) so they will be very similar mileage when towing.