Schloss means castle or chateau in German. Schloss Johannisberg and Schloss Rheinhartshausen are different wineries. Until the 1960s, Schloss Johannisberg was the undisputed premier grand cru of the Rheingau with Rheinhartshausen probably classified as a "super second." Both failed to keep up with improvements in quality although Schloss Johannisberg is still generally pretty respectable. The best wineries in the Rheingau right now are (IMHO) Robert Weil, Josef Leitz, Georg Breuer and Franz Künstler.
Schloss Johannisberg is one of the most impressive chateaux I have ever visited. If you are interested, pictures and some description are on my website
here. For pictures of Schloss Rheinhartshausen and some text, go
here.