Quill is good but not great. I looked at Quill extensively before embarking on writing Ritzy. Its collaboration features are minimal -- as far as I know there is no way to do collaboration across two browsers i.e. there is no working and/or available server-side component to coordinate the OT deltas produced by Quill (both editors have to be on the same page, which is somewhat pointless). I believe the authors were moving towards doing this via compatibility with ShareJS, but never quite got there. Offline support will be a pain to add with OT.

ProseMirror looks cool. I wish @Marijn the best with it! Not sure about ProseMirror's offline capabilities. I hope he adds multiple cursors and selections like Google Docs (and Ritzy).