Empty 450/550's are brutal. Air suspensions solve that for the most part. A typical loaded truck (sprung) will be super smooth on the rear (unbelievable what can sit on the outside of the rack...like a partial pop can at the rear...and make it 40 miles home. But the drivers area will have a much more 'jarring' ride. i.e. don't try to write something on a pad...it will look like you have Parkinson's or drank to much coffee. It's not a bad ride, but it's not plush. Just isn't much weight on the front typically, and add the wheel style (19.5's) and the result resonates more.
With stock springs, when loaded, they ride firmer than a 350/3500. But that's partly because most of those are maxed out/overloaded.
We're over 19000 solo, usually 27, 28000 when pulling our trailer. Run the numbers, but I bet you'll find that the 450/4500 would have less room to wiggle. I know very few folks who end up weighing less than they thought, carrying less than they think they will, finding that they 'over did it'...
I haven't had a 450 since about 1996...everything since, and now (have three currently) is 550. It's about the spring pack..rest of the truck is pretty much the same.