I'd definitely recommend staying at Gerd's place in Vienna. The flat is simple but roomy, extremely quiet, in a great location. All amenities can be found around (supermarket, transport etc), and the Belvedere is just a stone's throw away.
Even though it was quite hot in Vienna, around 27 celsius degrees, it felt much cooler in the flat (around 22 degrees) since the walls are very thick so they isolate from the heat. Gerd has been very helpful during my stay, and was very keen to help and welcome me.