My Mom lives about equal distance from EWR and PHL, so I have the same dilemma when flying in. Among the factors I personally consider for each airport:

• The drive to PHL (mainly down 70) is more relaxing than the drive to EWR and generally subject to fewer traffic delays.
  
• Fares into PHL seem to be lower compared to EWR (mainly looking at flights on WN and AA), but lately I've seen less of a difference.
  
• Flying from Chicago, I earn more miles to EWR than PHL.
  
• PHL seems to have more delays than EWR.
  
• Car rentals usually seem more expensive at PHL.
  
• EWR seems to have more public transportation options.
  
• Security is less than ideal at both airports. Focusing on the AA and WN concourses, neither is designed to handle the post-9/11 security needs. (Though AA at PHL has improved and I know they're working on WN improvements at PHL.)

The big question you didn't answer is: What destinations do you most frequently fly to? That can have a big influence on your final decision.