Disney World chose to negotiate with Equity many years ago. It is a more respected and has a bigger membership than AGVA. Disneyland chose AGVA, for reasons I don't know. Being a member of both, I can assure you Equity has a lot more clout and can play hardball. This translates to both positives and negatives for parks/CMs. It translates to performers not being taken advantage of financially (if the acrobats are non-union in DLR then they are most likely being taken advantage of) but also translates to making parade choreography less intricate since they won't let Disney get away with paying less to do more.
Universal entertainment dept is 100% union free, which means they can do what they want. Again, there are positive and negative results to that arrangement. Side note: Tokyo Disney is also union free.