When to Use SOAP vs REST WebService

RESTFul Web Services serves JSON that is faster to parse than XML
This so called "benefit" is related to consuming web services in a browser. RESTFul web services

REST is built for the Web
REST style architecture is optimized for the modern web architecture.

Caching

When using REST over HTTP, it will utilize the features available in HTTP such as caching, security in terms of TLS and authentication
Let's discuss this with an example; we have a RESTFul web service to serve us some stock quotes when provided with a stock ticker. Stock quotes changes per milliseconds, if we make a request for BARC (Barclays Bank), there is a chance that the quote that we have receive a minute ago would be different in two minutes. This shows that we cannot always use the caching features implemented in the protocol. HTTP Caching be useful in client requests of static content but if the caching feature of HTTP is not enough for your requirements, then you should also evaluate SOAP as you will be building your own cache either way not relying on the protocol.

Security

Security is never mentioned when discussing the benefits of REST over SOAP. Two simples security is provided on the HTTP protocol layer such as basic authentication and communication encryption through TLS. SOAP security is well standardized through WS-SECURITY. HTTP is not secured, as seen in the news all the time, therefore web services relying on the protocol needs to implement their own rigorous security. Security goes beyond simple authentication and confidentiality, and also includes authorization and integrity. When it comes to ease of implementation, I believe that SOAP is that at the forefront.

Conclusion
For machine-to-machine communications such as business processing with BPEL, transaction security and integrity, I suggest using SOAP. SOAP binding to HTTP is possible and XML parsing is not noticeably slower than JSON on the browser.For building public facing API, REST is not the undisputed championOne good thing about working with REST is that application do not need a service contract a la SOAP (WSDL)

REST works really well for are:

Limited bandwidth and resources; remember the return structure is really in any format (developer defined). Plus, any browser can be used because the REST approach uses the standard GET, PUT, POST, and DELETE verbs. Again, remember that REST can also use the XMLHttpRequest object that most modern browsers support today, which adds an extra bonus of AJAX.