SOAP UI - pros and cons

Our QA organization  is currently working on exploring how to test  REST API. we will be building our first API's so trying to explore on what API testing tools are in market ? 

 In process of research we have nailed down to SOAP UI and Postman tools.  But looks like we have several product versions -SoapUI, SoapUI Pro, SoapUI NG, SoapUI NG Pro and ReadyAPI.

Can anyone explain or point us to right location where we can find difference between these product version and pros and cons ?