Well, I'm probably in the minority, but I'd choose Animal Kingdom for a few reasons -
1. It's my favorite park!
2. There are lots of things there that will appeal to all ages - the safari and the gorilla and tiger walking trails, the Festival of the Lion King show, etc. plus a few things that your older one will love (Dinosaur, for example) and that your younger one will love (The Boneyard, for example).
3. It's easily seen in one day without too much rushing and without feeling like you're skipping over things.