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.

Almost any format is superior to Pickle or Marshal for interchange.

If you are passing around data from Python, to Javascript to even Flash, JSON is the best. Minimized parsing, smaller subset of YAML and KISS methodology. It doesn't have the verboseness problems that xml has but isn't as compact as binary data. But JSON converts to javascript, actionscript and python objects with almost no effort. It is actually not bad as a socket messaging format over xml or binary as well.