A few comments.

1.) I'm Viennese (grew up both in Vienna and New York)
2.) Demel's Sachertorte is better than at the Sacher but many off-brand Kaffeekonditorei have an even better product.
3.) The Palais Hansen Kempinski is mysteriously not on this list and it's as good if not better than the PH and RC and it's much more Viennese
4.) The Bristol v. the Imperial is a difficult decision. The best way I could explain it is my mother loved the Imperial but my grandmother (far more traditional Viennese Witwe) preferred the Bristol on return visits.
5.) Depending on how long your stay is, stay two nights perhaps at two different hotels for a different Viennese experience.