Content Repository is a standards-based, fully featured repository, included in all the editions of the Adobe Digital Enterprise Platform and Solutions. You can use it to store and access content, execute rich set of content services like search, authorization, etc. As such, it's ideally suited for all the content/document/asset-related needs of your applications. It offers the JCR API, plus access through RESTful interface and WebDAV network drive (among others). Additionally, managing content in the Content Repository makes it immediately availble for all the solutions that run of the platform.
In essence, Content Repository is your go-to place to store and manage content in the Digital Enterprise Platform/Solutions. It is well suited to cater to your use case.
The ECM Plugins (Connectors) are used if you have existing LiveCycle document services, and you have one of the ECM systems in your infrastructure used as archive. It allows you then to connect to such a 3rd-party system, and retrieve document for processing, as well as store them back for archiving.
