The 2018MY 6.2 is rated to tow about 1800 more lbs than the 5.3. Mechanically, the major differences from a 5.3 SLT with Max Tow to a 6.2 Denali are going to be engine, transmission, final drive ratio, and suspension. The Denali has a lower payload which tells me the suspension is not likely the reason for the difference in tow capacities. I doubt the engine is the significant difference either. Gear ratios are obviously different but the increased power of the 6.2 likely makes up for that. I'm guessing it's the transmission.
I've towed that much with the older 5.3 (with 4-speed?) and it was sufficient. I don't think the 5.3 should be an absolute deal breaker for you if you find the perfect one, but I think the 6.2 (and 10-speed) is the better fit for you. No one ever said they wish they had less power while towing. And one day you might want to tow something a little heavier anyway.
Also gotta say if you've got 5 or fewer passengers and can deal with a little extra length, jumping up to a 2500HD pickup would be a much better tow machine/experience overall.
Either way, don't forget weight distribution hitch, sway control, the right mirrors/cameras, etc. That stuff is going to matter more than the power difference between the 5.3/6.2 when it comes down to it.
(I think you posted in the wrong section of the forum--should be in the 2015-2020 section instead of 2021+)
