I'm neither a subscriber nor a native German-speaker, so no guarantees. The Rundschau is the normal monthly update on new issues, with several article included. I think the Plus is a quarterly-ish extra issue that, in addition to listing new issues, has a few longer articles on select topics, and more pictures of new issues than the normal Rundschau.
See for example, the Table of Contents from the August Rundschau and Rundschau Plus.

