If you can afford a 2007+ GMT900 Denali, that would be a very noticeable improvement in power and 6-spd transmission gearing vs. 4. The AWD has been very reliable since its introduction and shows no specific sign of extra wear or handling compromises when towing.
Keep in mind the payload of the Denali is relatively low for its tow rating. It you are considering towing 6k+ dry, you should consider the 3/4 ton Suburban/Yukon XL.