Can anyone recommend a good MP that is accessible through Philly's public transit? Live in the city so don't have a car but would like a place not so close to home... Thanks.

Depends on whether or not you are looking for full service. If you are, all of the good ones are in the city. There should be plenty that are discreet enough for you. However, if you are not looking for FS, my personal favorite is Ridge Spa which should be accessible through public transit.