I definitely recommend staying at Qing's and Felix's apartment. Conveniently located, U-Bahn 6 und Straßenbahn 5 right next to the apartment. Walking to the city centre is also an option. A lot of shops around.
On the downside, apartment can get really hot (it was over 30 degrees outside though), but a fan was provided.
I really recommend staying here on your sightseeing trip to Vienna.