Key Differences between XML and JSON

Comparing JSON to XML

Both JSON and XML can be used to represent native, in-memory objects in a text-based, human-readable, data exchange format. Furthermore, the two data exchange formats are isomorphic—given text in one format, an equivalent one is conceivable in the other. For example, when calling one of web services , you can indicate via a querystring parameter whether the response should be formatted as XML or JSON. 

   Therefore, when deciding upon a data exchange format, it's not a simple matter of choosing one over the other as a silver bullet, but rather what format has the characteristics that make it the best choice for a particular application. For example, XML has its roots in marking-up document text and tends to shine very well in that space (as is evident with XHTML). JSON, on the other hand, has its roots in programming language types and structures and therefore provides a more natural and readily available mapping to exchange structured data. Beyond these two starting points, the following table will help you to understand and compare the key characteristics of XML and JSON.

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Key Characteristic Differences between XML and JSON

Characteristic	XML/JSON
Data types	Does not provide any notion of data types. One must rely on XML Schema for adding type information. Provides scalar data types and the ability to express structured data through arrays and objects.
Support for arrays	Arrays have to be expressed by conventions, for example through the use of an outer placeholder element that models the arrays contents as inner elements. Typically, the outer element uses the plural form of the name used for inner elements. Native array support.
Support for objects	Objects have to be expressed by conventions, often through a mixed use of attributes and elements. Native object support.
Null support	Requires use of xsi:nil on elements in an XML instance document plus an import of the corresponding namespace. Natively recognizes the null value.
Comments	Native support and usually available through APIs. Not supported.
Namespaces	Supports namespaces, which eliminates the risk of name collisions when combining documents. Namespaces also allow existing XML-based standards to be safely extended. No concept of namespaces. Naming collisions are usually avoided by nesting objects or using a prefix in an object member name (the former is preferred in practice).
Formatting decisions	Complex. Requires a greater effort to decide how to map application types to XML elements and attributes. Can create heated debates whether an element-centric or attribute-centric approach is better. Simple. Provides a much more direct mapping for application data. The only exception may be the absence of date/time literal.
Size	Documents tend to be lengthy in size, especially when an element-centric approach to formatting is used. Syntax is very terse and yields formatted text where most of the space is consumed (rightly so) by the represented data.
Parsing in JavaScript	Requires an XML DOM implementation and additional application code to map text back into JavaScript objects. No additional application code required to parse text; can use JavaScript's eval function.
Learning curve	Generally tends to require use of several technologies in concert: XPath, XML Schema,XSLT, XML Namespaces, the DOM, and so on. Very simple technology stack that is already familiar to developers with a background in JavaScript or other dynamic programming languages.

JSON is a relatively new data exchange format and does not have the years of adoption or vendor support that XML enjoys today (although JSON is catching up quickly). The following table highlights the current state of affairs in the XML and JSON spaces.

Support Differences between XML and JSON

Support	XML/JSON
Tools	Enjoys a mature set of tools widely available from many industry vendors. Rich tool support—such as editors and formatters—is scarce.
Microsoft .NET Framework	Very good and mature support since version 1.0 of the .NET Framework. XML support is available as part of the Base Class Library (BCL). For unmanaged environments, there is MSXML. None so far, except an initial implementation as part of ASP.NET AJAX.
Platform and language	Parsers and formatters are widely available on many platforms and languages (commercial and open source implementations). Parsers and formatters are available already on many platforms and in many languages. Consult json.org for a good set of references. Most implementations for now tend to be open source projects.
Integrated language	Industry vendors are currently experimenting with support literally within languages. See Microsoft's LINQ project for more information. Is natively supported in JavaScript/ECMAScript only.

Note   Neither table is meant to be a comprehensive list of comparison points. There are further angles on which both data formats can be compared, but we felt that these key points should be sufficient to build an initial impression.

Source from  : http://msdn.microsoft.com/en-us/library/bb299886.aspx