YAML is strictly a superset of JSON in its expressiveness, since it allows tagging of serialized objects with a type name. JSON reduces everything to maps, arrays, and scalars, so any type information has to be encoded in (or inferred from) the structures themselves. I also find it to be a bit easier to read, and so prefer it for configuration files or console dumps of data structures.

However, YAML isn't quite as well-supported in the standard libraries of various programming languages, most notably Javascript. Which one to use depends largely on how heavily you expect browsers to consume your service output.