There are two main differences -- width and pressure.
1. All Chirp Wheels are 5" wide. This width is perfect for putting the pressure where it is needed along the spine and between the shoulder blades. A foam roller just steamrolls the whole area with no real focus.
2. Chirp Wheels come in 3 sizes (diameters) -- 12" 10" and 6". Due to the size change, the area in contact with the muscles at any given point provides a different amount of pressure. Think wearing high heels in the snow vs. actual snowshoes.
Check out this video to see more.