I only know people that have been to Featherdale (Wildlife Park) and haven't been there myself. I've been to the zoo and the Wildlife Centre though.
From what I understand, the main attraction for Featherdale is the opportunity to walk among the animals, I suspect mostly kangaroos and wallabies. As you noted, Featherdale has a larger area to allow that to happen.
Maybe someone that has been to Featherdale can share more information...?