My nitto 18s are rated for 4080 lb each. That was the highest I'd ever seen on an Lt tire, or an st for that matter.
But if I could get a new 3500 truck with 20s with a high enough load rating I would. But if I did buy a new one, it would probably be a dually, so 20s are out.
Not much difference in ride wheel wise. The tires are where the difference is....its the stiffness from the load rating and tire make up. My 18s are ROUGH, but DWs p rated 20s are super smooth.