Advantages of XML

• Near ubiquitous support in a wide array of languages and frameworks. More likely than not there's already a tool out there to help your extract information from an XML response.

• It can adhere to a concrete schema if so you choose. Once it validates, you can say it's correct and start parsing.

• Namespaces allow you to divide the XML.

Advantages of JSON

• Lightweight in comparison to XML. Fewer characters = smaller time going through the internet tubes.

• Easier to handle with Javascript if you need something for a web application.

Native XML databases (eXist, BaseX, etc.) are optimized for executing XPath and XQuery requests over collections of XML documents. However, my experience with them is very frustrating as their performance and scalability are very poor.

I would always choose an object-relational DB instead, i.e. MySQL, or a highly distributed database (e.g. Cassandra). Even if you need to store complex XML documents that do not map gracefully into tables you don't need a native XML DB unless you need to query the documents with XPath/XQuery expressions. As far as I know mongoDB has a good performance, good scalability, and works with JSON, which is the current trend. However, I disagree with the most part of advantages that people give to JSON against XML. First one uses to be about the "weight" of the instances, but neither XML nor JSON are efficient for data transmission or storage unless you compress the documents, and in that case they are equivalent. XML is a mature standard with a schema language (XSD), a query language (XPath/XQuery) and tons of APIs and tools. JSON has more intuitive programming tools and works better with Javascript. JSON instances are more human-friendly (both for reading and writting). My choice? are you sure you cannot do it with MySQL?

i haven't ever used xml databases but i think that the xpath and xquery expressions are simple a very usefull. But in my point of view i would like to try with mondodb because is very simple to use with python and i don`t know how to programm the xml databases.

I am trying to develop a service to share information between users of my application and i didn't want to use relational systems because my data structure is quite simple.

Besides i think that mondodb y easy to program with python because the json documents are very easy to manage with python.

In the end it will always be a question of performance and the need for advanced queries. XML databases might have the advantage of being easier to query, but my guess is that they perform worse than mongoDB (which btw stores its data in BSON-format: http://docs.mongodb.org/manual/reference/glossary/#term-bson).