First, choosing a data structure(xml,json,yaml) usually includes only a readability/size problem. For example
Json is very compact, but no human being can read it easily, very hard do debug,
Xml is very large, but everyone can easily read/debug it,
Yaml is in between Xml and json.
But if you want to work with Javascript heavily and/or your software makes a lot of data transfer between browser-server, you should use Json, because it is pure javascript and very compact. But don't try to write it in a string, use libraries to generate the code you needed from an object.
Hope this helps.