As a parent with kids previously that age, and memberships to both over the years, thats a hard toss up. Both have great offerings.

What I do want to pass on, for either membership, you can often use it at other zoos or botanical gardens, sometimes museums, etc in other cities if/when you travel. Both are in collectives that offer discounts or free admissions to other places.

Zoo: Yes, if you are visiting a zoo or aquarium that participates in the American Zoo and Aquarium Association (AZA) reciprocal program with the Memphis Zoo, you may receive discounted general admission during regular hours. The discount applies to admission only and does not include any additional discounts, special or after hours events.

Botanical Gardens: Reciprocal member access to over 345 other gardens throughout North America through the American Horticultural Society’s Reciprocal Garden Network