I have both in box and flat and nothing comes close to how smooth a double axle is, period. The weight capacity has nothing to do with it.
Think of it like this, your going over a very rough R/R track, every little ridge is hitting that single wheel and that single wheel will bounce, now imaging two wheels where one is not bouncing at all times.
As an example I have a Dixie Chopper I use to haul south for the other house. With the flat trailer I most definitely had to strap it down or it would exit the trailer. But in the blue box trailer all I had to do was set the parking brake, it was always right where I parked it when I got to where I was going. Same goes for many other pieces of equipment I hauled back and forth.