Have you read the specs for both? You should read the JSON spec. It will take you ten minutes. The YAML spec (http://yaml.org/spec/1.2/spec.html) may take you a little longer. . .
S-expressions will take you ten seconds still nobody uses them. YAML is made to simplify the lives of the reader and the writer, not the parser. Let the parser suffer, let the coder rejoice. You pick your own level of suffering, I pick mine. Deal?
What language you use doesn't really matter much when you can easily transform one into another. I think that's exactly the case with JSON and YAML.