why json vs xml
Let's compare XML and JSON on the attributes that the XML community considers important. From http://www.simonstl.com/articles/whyxml.htm. XML is simpler than SGML, but JSON is much simpler than XML. JSON has a much smaller grammar and maps more directly onto the data structures used in modern programming languages. JSON is not extensible because it does not need to be.
Syntax: JSON syntax is quite light and definitely less verbose than XML, with all of its start and end tags. When it comes down to pure byte-size, JSON can represent the same data as XML using fewer characters. Weight: Since JSON syntax requires fewer characters, it is lighter on the wire than XML. The question is if this really matters. Any large data set is going to be large regardless of the data format you use.