I would scratch 15219 off your list. It is not a safe area whatsoever to live in or even near, let alone drive through.
15213 is Oakland. Oakland has a "north" end and a "south" end. Some points in South Oakland are not exactly walk-able. Squirrel Hill covers a large area, where again, some points are reasonable walks, while others are not (especially in the snow!).
The East End busway is a popular mode of transportation when traveling to town from the East. The "T" is a popular way to get from some locations in the South Hills. Check out http://www.portauthority.org for more info.
Many commuters by car use Shenley Park for parking and walk to class at Pitt, Chatham or CMU.
Hope this helps!
It all depends on which university you are looking at. CMU and Chatam University are on opposite sides of Oakland. While the University of Pittsburgh is in the heart of Oakland.