My recommendation would be to stay as close to to City Hall as possible within your budget so that you are easily walkable to majority of things you want to see and do. Other areas to consider are Fishtown and the waterfront area.

Tips for where to stay:

• For easy accessibility and transportation to many areas in Philly, stay within walking distance to one of the subway lines
• If you plan to stay in West Philly across the Schuylkill River, do not stay somewhere further west than the 40th Street subway stop. Safety significantly decreases once you venture past this area