Question What is the best use?
In Ajax, I use JSON and honestly I do not understand why xml?
What is global differences except for building ???
Pros, cons.

Answer 1

Previously, there was no JSON , used XML , now many technologies are so supported for alternatives, many open API give information too in XML .
Here is the article on this issue: json and xml. What is better? .
The author told in detail about the differences in the formats JSON and XML .

UPD:
With reference late ..

Answer 2, Authority 300%

I work in supporting banking software. Everything is on XML.
SOAP protocol is used to interact different services.
About the transition to JSON even there is no thoughts.)))
My opinion, to use what is responsible for tasks. If you see the C JSON solution, do with JSON. It is easier to work on the client.
If the task requires XML, Yuzai XML. For example, the interaction with the side of the system through SOAP.

Answer 3

The fact is that it happened historically. The XML format was introduced into the turnover by Microsoft and immediately they began to shove him in all their crafts. Moreover, as the top of creative thought.
With everything, it seemed, his “versatility”, it is very bad in the implementation of working with it and a very redundant format. Crafts, which could not even be born, at least somehow, normally comprehend and implement.
JSON is a more compact and understandable format.