I think it depends on the kid - but there are so many great things at Epcot for that age group, some often overlooked.
My kids loved Epcot since age 2 starting with the WS because they loved all the live entertainment. Favorites were the acrobats in China, Mo-rockin and Off Kilter - the latter mostly because it was a fave of mine!
The boat ride in Mexico is fun, a bit like small world at the end as is the Malestrom in Norway, another boat ride with a small drip (like POTC) and some friendly trolls.
The girls love making the mask and decorating it and adding to it in each country. They like to play on the viking ship in norway and see the trains in Germany.
In future world they like the living seas, all of the imagination pavillion and most of the land. They like the Innovations pavilion closer to test track esp the interactive games and toy making. I would say the only thing they didn't like at FW they were tall enough for was the energy adventure.
Just before he turned 4 in December 05 we took my nephew to Epcot, IIRC one full day and most every evening and he loved everything as well. He even rode Test Track a few times.
So if you find she is into the rides then stick to FW, if he likes people and live entertainment hit the WS. If she likes both then spend more time!!
Honestly, we spend most of our evenings at WS because a day at the parks overloads us all. Other than an evening at MK for wishes and one at MGM for Fantasmic every night of our vacation ends at Ecpot. Esp the WS, it just seems more relaxed but still that vacation fun we like.
Sorry so long
TJ