I would say at that age if you can afford it, do both. If you have to choose one, then I would choose the castle. I've never been to the Princess Breakfast at EPCOT, but I thought the food was much improved at the castle when we went this July and there's nothing like the atmosphere of the castle.
I think seeing the characters and getting autographs at that age are the most important thing for them. We look at it like a "that's all" thing, but to them it's everything. These are their heros and heroines. My niece is 7 and when she got back and people asked what her favorite thing was she said getting autographs and seeing the characters, (of course she named them all) and she did all the big rides RNR, TOT, Splash MT. Space MT. etc.
She had Cinderella sign her Cinderella Barbie doll on the leg and also bought a bride Minnie and had Mickey and Minnie sign the dress. Plus of course, everyone she saw had to sign her book!!! I would bet that she's going to remember and talk about who she saw and talked to, more than any attraction you'll take her too!!!