If you're going to spend a day in Epcot, I would suggest the Garden Grill.
It's buffet-style (they bring the food to you), character dining, and the restaurant rotates slowly (you get to see "Living with the Land" attraction at certain points). It's a really great atmosphere, and the food is AMAZING!
In the Magic Kingdom, I reccommend the Liberty Tree Tavern. Character dining is only for dinner, and since I have only been there for lunch (when it's regular table service) I can only comment on how great the food is. But, they serve the same food in the evening in buffet style (similar to how the Garden Grill works). The Turkey is to-die-for!
Enjoy your trip!