Considerations: A tool is only as good as the way it is applied to the body.
The same tool could be great for one person and cause injury to another- because of what the body’s tissue needed.
The Shape+ Firmness of the tool determines the surface area:
A Ball has a point (smaller surface area but more "in")
A foam roller has a broader surface area.
The hardness of the tool determines how "in" it goes...
Note: The ideal tools for the body have some “Smoosh”. The more rigid the tool, the more aggressive/sharp and possibility for injury. There should never be pain when using a tool. Intense sensation- if not a red flag- can be ok.
The Size: should be appropriate for the area you are working
Smaller balls might press on bones (create movement) in an individual foot joint, bigger balls can fit in the area to the side of the spine, while even bigger balls make a bigger movement into the quads. Rollers (foam or otherwise) either press into the tissue or the body can stretch around it. The body would also stretch around a large ball.