I've just tested both of them and got the same results. Why?
My intent is to generate server stubs for Google Java Endpoints Framework from OpenAPI specification. Is it possible?
First, I must say I do not believe that our intention is not facilitate the creation of new projects using these tools. These tools were built for migrating old projects and are not a recommended path forward.
But to answer your question:
endpoints-framework:clientLibs is an operation your run on your server side project (appengine). It just downloads a client library (zip) for you. You can process and use this downloaded library in another project to consume the endpoint.endpoints-framework:generateSrc is an operation you run on the endpoints consumer (or client). This is a helper that automatically includes the client library code in your project (it also downloads the client library), so you don't have to copy that code in yourself (or generate a library an import it in)