We’d recommend starting with the least amount of resistance. This is based on the fact that the stress relief really comes from the physical act of manipulating the material. It’s why the really squeezy slow-rise foam balls are popular. Younger children will need to begin with the ones that are the softest because they have less muscle strength than older kids. If the child is older, then let them choose which they prefer. Make sure to get a variety, as these are economical toys and can be bought in groups.