We are HUGE fans of Animal Kingdom and spent 2 1/2 days of our last trip there and did NOT do Everest, FOTLK or the Nemo musical. My kids can watch the animals and play in the dinosaur dig area for hours. They also have lots of characters. On the other hand, we spent a half day at MGM because there are so few things my kids (ages 6 and 4) like to do.
That said, if your family isn't zoo crazy, MGM has TONS of characters. If your kids like characters, MGM is the only place to find Little Einsteins, JoJo and Goliath and Cars Lightning and Mater. They also have a great Toy Story character area as well as lots of other chars. The parade is great, the Honey, I shrunk playground is fun. My kids don't like Little Mermaid or Playhouse Disney (EVERY other kid in the theater seemed to love it) and were terrified by the Great Movie Ride. They weren't crazy about Star Tours so we definitely skipped ToT. The great thing about MGM is how small it is. Oh, DS4 LOVED the new stunt show.
My kids hate to sit through shows but love to watch animals so AK is our choice.