Sarah and Frank's flat is absolutely a great location. Incredibly close to the metro station ( just opposite to the flat) and 2 mins walk to the tram stops. Besides, most of the tourist attractions are in the walking distance.
Speaking of the flat itself, spacious, clean and modern are the adjectives I will use.
Again, since the flat is inside the center, u can do Everthing u need, shopping, eating, public transportation. My wife and I enjoyed the Jazz at Reduta Jazz club. Again 150 m away from the flat.
In a word, choose their flat if u come to Prague