the 10 footer - can keep large Bamboos, eppies, or Cali Horn sharks for life. Or possible Smoothhound pups.
The 15 footer would be okay for Smoothhound adults(3-4'), or small requiem shark pups (2.5-3.5' - depending on the species).
Still for either the smoothhounds or the requiem pups - if possible a larger pool would definitely be better.