What's the difference?
Both awards are bestowed upon children's books by the American Library Association. The awardees must be American citizens or residents and have had their books published in the United States in the past year. In addition to the medals, there is also a shortlist of honorees each year.
The Caldecott Medal is an annual award given to outstanding American children's illustrators. It was established in 1938. Most Caldecott award winners are picture books. Notable past winners include The Polar Express (Van Allsburg), Song and Dance Man (Gammell), and Where the Wild Things Are (Sendak).
The Newbery Medal is an annual award give to outstanding American children's authors. Established in 1922, it was the first children's book award in the world. Most Newbery award winners are chapter books. Notable past winners include Bridge to Terabithia (Paterson), Number the Stars (Lowry), and Walk Two Moons (Creech).
Which medal is better?
While all are entitled to their own opinions, I believe the medals are incomparable. Both of them require literary excellence for children, which can be a subjective decision. In discussions of children's literature, the illustrations and text both play strong roles to tell a story. I have often heard the argument that simple picture books are unworthy of these medals. However, how many children have been turned on to reading by simply looking at the pictures? That is the earliest form of literacy.