Is there a smoother ride

I had them both . I think the dual axle rides a little smoother, but the single axle on a 21 footer is plenty adequate if done right. I had blow out and the single axle just laid down without any sway or issues whatsoever. In addition cheaper tire/wheel bearing, brakes maintenance and less on the tolls if you live or travel in heavy toll laden areas like northeast.You pay per axle. The nice thing about dual is could drive a ways if you get a flat on one of em before changing it out. The single has to be done immediately.