SOAP vs REST. Which one to choose when?

1. REST naturally fits for Web/Cloud API's, whilst SOAP fits for distributed computing scenarios.
2.  JSON is a widely-recognized and simple standard for data exchange, and is easily read by browsers and client code, which is why most RESTful API's (Facebook and Twitter API is a good example of RESTFul API) offer JSON.
3. The RESTful Web services are completely stateless (every request is separate and not dependent on earlier request). This can be tested by restarting the server and checking if the interactions are able to survive. While, SOAP service are stateful (each request is dependent on earlier request. Hence, sometime server has to persist few request to get few info. for future  request).
4. REST is meant for caching/scalability of application. Whilst, SOAP is meant for security/contract between Client/Server.
5. REST-based implementation is simple compared to SOAP.Programming in REST is easy compare to SOAP.

Problem with SOAP (Simple Object Access Protocol):

1. Works with XML to exchange information between entities.The XML used to make requests and receive responses in SOAP can become extremely complex. In some programming languages, you need to build those requests manually, which becomes problematic because SOAP is intolerant of errors. However, other languages can use shortcuts that SOAP provides; that can help you reduce the effort required to create the request and to parse the response. In fact, when working with .NET languages, you never even see the XML. Part of the magic is the Web Services Description Language (WSDL). This is another file that’s associated with SOAP. It provides a definition of how the Web service works, so that when you create a reference to it, the IDE can completely automate the process. So, the difficulty of using SOAP depends to a large degree on the language you use. In short, Many developers found SOAP cumbersome and hard to use. For example, working with SOAP in JavaScript means writing a ton of code to perform extremely simple tasks because you must create the required XML structure absolutely every time.While, REST provides a lighter weight alternative. Instead of using XML to make a request, REST relies on a simple URL in many cases.
2. Unlike SOAP, REST doesn’t have to use XML to provide the response. You can find REST-based Web services that output the data in Command Separated Value (CSV), JavaScript Object Notation (JSON) and Really Simple Syndication (RSS). The point is that you can obtain the output you need in a form that’s easy to parse within the language you need for your application.

Deciding Between SOAP and REST

SOAP is definitely the heavyweight choice for Web service access. It provides the following advantages when compared to REST:

• Language, platform, and transport independent (HTTP / SMTP / FTP) (While, REST requires use of HTTP)
• Works well in distributed enterprise environments (REST assumes direct point-to-point communication)
• Standardized
• Provides significant pre-build extensibility in the form of the WS* standards
• Built-in error handling
• Automation when used with certain language (like .Net) products.
• Mainly meant for enterprise application (bank website, organisation website etc.) since SOAP provide advantage of security feature (WS-Transaction specification).

REST is easier to use for the most part and is more flexible. It has the following advantages when compared to SOAP:

• No expensive tools require to interact with the Web service
• Smaller learning curve
• Efficient (SOAP uses XML for all messages, REST can use smaller message formats)
• Fast (no extensive processing required)
• Closer to other Web technologies in design philosophy
• Support more interoperability compare to SOAP. Check here.
• Mainly used for Internet facing application (Facebook, Twitter) since REST brings advantage of scalabilty and speed.Check here (After all that information, aren't you telling me that REST is good for Internet-facing applications, and SOAP for enterprise applications?)

Reference document (best article to understand SOAP and REST in few min.) Thanks to John Mueller.

http://blog.smartbear.com/apis/understanding-soap-and-rest-basics/