Between the two I enthusiastically say Epcot. We have 3 sons and on both trips they enjoyed Epcot (first trip they were 4, 4, & 7), including the Kidcot stops in World Showcase. Some other favorites were the Seas with Nemo ride, Turtle Talk with Crush (so fun!), the Seas acquarium, the Imagination play area, the cool fountain near the Fountainview Cafe in WS, the 3 Caballeros ride at Mexico, the miniature train in Germany, the huge troll in a shop in Norway, and seeing the juggler in Italy. (The balancing act in France is also fun.) There's a lot to see and nice gardens to run around in a bit if they need a little play time.
We also live near a good zoo at home and many of the animals at Animal Kingdom weren't new to us. Plus, there aren't many rides for young kids. We just didn't enjoy it as much. (Also, in the summer it gets very hot.) Next time we're skipping AK. That said, if you do go to AK, I'd personally recommend going slowly through the Maharajah Jungle Trek & Pangani Forest Exploration trails and asking questions of the castmembers about the animals. They have a lot to share about them. That was what made it interesting for us.
...But for us, Epcot is so much more fun, especially if you make an effort to do the things your children would enjoy. There's lots to do.
I just found some fun ideas in this article on Epcot:
http://allears.net/tp/ep/issue099.htm
Have fun wherever you go!