The wall ball publishes 6 kg at 35 cm and states that it absorbs impact
and does not bounce.
Those are the two facts that define a wall ball: the large diameter is what
makes a repeated chest catch possible, and the non-rebound is what makes it
predictable.
The same listing names no material at all, which the review page records.
The pilates ring
Approximately 39 cm — the widest on its shelf — described as soft and
hard-wearing. No material, no mass, no hoop thickness.
On a shelf where nobody publishes a resistance figure, the diameter is one
of only two things that can be compared.