In the past for in ground pools, companies used vermiculite (which supposedly move around a little bit). They started using a stronger base (sand/cement mix). If no one is jumping and doing cannonballs from a ladder or tree, the Pool Crete (which is probably a harder base is best). If not, then stick with the sand or vermiculite, which is a softer base.
I believe there is no best or right choice, just different options. Good luck and keep us posted. Once completed, please add your signature so in the future if you have any questions, we can be of help.