We have stayed twice at K&T Boardinghouse (59 E for small double without bath - I think still 69E for very nice double room with bath) on Marianhilfer Strasse (just came home last week). With a transportation pass (24 hour or 3 day), it's easy to get everywhere (and can be walked if you are ambitious). It is just 2 stops from the Westbahnhof on the metro(and on the bus route that goes to the Sudbahnhof), and on the metro line to Wien Mitte (which is where the CityAirportTrain comes in), so handy to the trains. They don't do breakfast, but there are several stores and bakeries in the same block (plan ahead for Sunday - nothing is open).