If you look for a cheaper Marriott place, try the Renaissance Wien (Category 4), three weekend nights for € 96,--/night (breakfast for two included) during the VISA Summer Promo. This hotel is located opposite to the underground station Meidling Hauptstraße (U4). From here, you have two stops two Schönbrunn, four stops (other direction) to Naschmarkt (U4 Kettenbrückengasse) and five to the opera house and Kärtner Straße (U4 Karlsplatz).
BTW.: On not to busy weekends Marriott Vienna could be a good option. This hotel is sometimes cheaper than the Renaissance Penta Vienna Hotel.