Actually, I got off at Russelheim (just before the Opel factory stop) and waited for the next S8 to Mainz Hbf (the RE that was supposed to come before that train failed to show up). It would have been a long walk across the Rhein with bags.

I just looked at the online RMW.de timetable and it seems that the S8 runs at a certain time M-Sa and the S9 runs at the same time on Su. There wasn't that distinguishment on the yellow timetable on the station platform as far as I could discern.