Few days back, one of my team member asked me  – Which one should I use? SOAP or REST? 
I believe this is a very fundamental question, but definitely a very very important one because it will define your project’s success.
If you want to learn more about REST, please have a look at this one – really good starting point – “How I explained REST to my wife?”

To know more about Salesforce Integration using SOAP API, please check –
We should not compare SOAP and REST as they can’t be compared directly. The reason why I am telling is that because SOAP is a protocol while REST is an architectural style. People normally used to say any HTTP API that isn’t SOAP, REST and that is why the confusion.
The main difference between SOAP and REST is the way of coupling between server and client implementation. A SOAP client works similar like custom desktop application and normally tightly coupled with server. So if any side changes anything, the entire contract will break.
Whereas REST client is more like a browser. So it is more generic, loosely coupled and it knows how to use a particular protocol to communicate with server.
With the above basic, let me post a comparative study on SOAP Vs REST. I hope this will help you to get a quick overview and also help you to make decision.

When we are doing this comparative study, we should consider architectural constraints also. Let’s have the details below –

Please provide your comments. Thanks.