I've been here searching all morning for a step and trying to see what the differences are,YOu hit it on the head. The length. Get the biggest one you can. I have this little one that I am trying to convince myself is okay but, I really do want a larger one to hold my body so can do the chest presses on it with ease. Or, I can use my Stability ball and hold off on the larger step so I can purchase a DVD. TODAY!!!