James Clark has a great post on XML and JSON. But I have one point to pick at.
"JSON's primitive datatype support is weak."
Next, "the set of primitive datatypes is not extensible". But this is true for XML itself. Until you mix in XML schema. It's schema that allows you to interpret a string in XML as some sort of other data type. JSON has no standard schema. Yet. It's easy to imagine though. Defined in JSON, of course.