There is no one answer since there are various needs and preferences. Someone prefers quiet and green neighbourhood and is able to commute extra 15 minutes, someone else is keen on city lifestyle and downtown would be his natural choice. Othough various aspect should be taken under consideration some general rules can be useful in the decision making process.
Singles or couples without children usually prefer either close distance to the office or downtown of the city. For them Środmieście (city center) or Powiśle (river bank district) are usually preferred destinations. Mokotow is also nice residential district where you can find nice apartments. In particular the area near Pole Mokotowskie park is attractive for those who prefer active lifestyle – jogging, roller skating or biking are easy in this neighbourhood.
Families with children usually prefer locations with easy access to international schools. Most of them is placed south of Warsaw. Preferable districts for those are Mokotow, Wilanow or Sadyba. The last one is very convenient for those with children in the British School and Lycee Francais de Varsovie since both of them are in walking distance to this area. Konstancja housing estate in Konstancin is in walking distance to American School of Warsaw. German School is placed in Wilanow and any location within this district shall be convenient for everyday life.
Don’t get confused when you talk about Konstancin.  It is in fact a small town near Warsaw and most of it’s area is covered by woods. It is a good choice for those who prefer country side lifestyle but is 45 minutes from the city centre. Within the borders of Konstancin there is also American School of Warsaw campus. It is located closer to the city (30 minutes from downtown) and it is not in the woods. There is Konstancja housing estate – a nice, closed compound in walking distance to the American School of Warsaw.