There is a new article on IBM developerWorks titled "XML or JSON: Guidelines for what to choose for DB2 for z/OS". It has been written by two very experienced technologists with DB2 background. Though the comparison and the many code examples for how to use either XML or JSON are for DB2 for z/OS, most of it also applies to DB2 LUW. So I recommend reading the article regardless of which "camp" you are in.
At the end of the XML vs. JSON comparison is a resource section with a list of good papers and documentation. In my blog's page on DB2 & pureXML Resources you will also find additional material on that topic.
With that to read enjoy the weekend!