Cant really comment on the other areas, but I love living in the Kreis 4 (western part towards the Hardbrucke). A short walk or ride to everything, close to motorway access if driving and a quick sport cycle to get up/over the Uetliberg. Rent (or at least ours) is cheap, I have to multply my rent by 2 or 3 to get the monthly rents I hear quoted by others around here. Though I think I have lucked out in this regard.

Thought I'd post the homegate.ch map to help visualise those Kreis numbers.