We also use Avistar--I think it's the Essington Ave exit. (Sorry--it's one of those things, I can drive there, but can't give you directions)
PHL is still about teh worst airport in the USA as far as the length of time for screening, etc. They just don't ahve it together yet.
Make sure to give yourselves two hours. That's not two hours from your house, not two hours from the park and ride, but two hours from the time you walk through the doors at the airport. You could well need every minute of it.
If you have e-tickets, do a self check in using the kiosk, it's almost always a major time saver, or check in over the Internet before you leave home.
Anne