Density decides impact energy. At a given velocity a denser particle arrives with more energy and cuts deeper, which is why carbon steel and stainless clean aggressively and aluminium, zinc and copper do not. Choosing a light medium is not choosing a weak one; it is choosing an appropriate one for a soft substrate.

Hardness decides how the medium wears rather than how hard it hits. A medium softer than the substrate deforms instead of cutting, which is precisely what a deburring application on a casting wants. A medium harder than the substrate cuts, which is what descaling steel requires.

Compatibility decides what is left behind. Any medium embeds some of itself in the surface it strikes, so the question is whether that residue matters. On steel it does not. On aluminium, magnesium, zinc, copper or stainless, ferrous residue rusts, stains and can start galvanic corrosion.

The practical rule is to match the medium to the substrate first, then choose shape by whether the job is cleaning or peening, then size by the smallest feature that must be reached. Price enters last, because the price that matters is cost per part rather than cost per kilogram.