Why does an indoor softball have to be different than an outdoor softball? We use the same balls regardless of our location, other than dimple balls that are required from some pitching machines.
It depends where we are at and where the player is. If our player in a more general area they are required to use a softie. Like DD goes to an indoor pitching clinic, with all that is going on they require her to use a softie.
When DD 3 was first learning to pitch, I was able to sometimes take her to a local HS gym to practice. We were told we had to get indoor softballs to reduce the damage to the gym. Another benefit: the indoor softball hurt less when a bucket dad gets hit.
I've used a lot of different brands and have found the Jugs Softies to be the best. We have to use them in the gym. They feel just like real softballs except when they hit you...and they bounce more like a real softball than anything else we've tried.