Your first priority should be to enlist the help of the right agent who can spot a good deal and knows the purchase and rental values in the area.
Like Bruce said, that is a difficult question to answer as a whole. But what makes it most difficult is to answer for the entire city of Philadelphia.
Real estate in many ways is very local. So what is going on in your neighborhood? Are prices continuing to drop? Is your neighborhood on the decline? Is your neighborhood association active in improving the neighborhood? Are homes being renovated or built in your area?
I think those questions will help you arrive at a more definitive conclusion.
Hope that helps. Let me know if I can be of anymore help.