It sounds to me like you are thinking something like: if they use XML, they'll have a well defined schema and will follow standardized XML types. But if they use JSON they may not have a well-defined schema at all, and may not follow any sort of standardized formats.
But to my mind, whether they have a well-defined schema and follow proper datatypes really has very little to do with the choice of XML or JSON.
But to my mind, whether they have a well-defined schema and follow proper datatypes really has very little to do with the choice of XML or JSON.