I've been to both and enjoyed both. Chef Mickey's is louder and I love the atmosphere with the monorail running through. If you are doing several character meals I'd go for Chef Mickey's (you get Mickey, Minnie, Chip, Dale, and Goofy). If I was just going to do one character meal I'd go with 1900 Park Faire because of the variety of characters (Mary Poppins, Pooh, Tigger, Mad Hatter, and Alice). 1900 Park Faire is quieter and we preferred the food there.
You didn't ask but our favorite character meal however, is Crystal Palace. We love getting into the park early to take pictures on Main Street and the Castle with no one else in them and the restaraunt is lovely. We've always received good food, good service, and good character interaction there. Plus we love the Pooh characters.