Christina and Bellal's place was perfect for my trip to Vienna. I particularly appreciated how private the space is, despite the large windows that let in plenty of light (important during the winter months when I stayed there).
Whilst not located in the middle of the city, the U-Bahn will put you in the first district in 15 minutes. But there's also the option to go for a walk in the hills, with easy access to the local wineries, for a change of pace (which would particularly nice in the warmer months)
Would happily stay again next time I need a place in Vienna.