NYC- Hoboken or Jersey City.

Both- Live on the Northeast Corridor NJ Transit Line. Not sure what the Trenton Bound is like in the morning. Princeton has plenty of express trains to NYC so you only have a few stops.

Somerville is also not a terrible spot. Access to the Train and Reasonable access to Philly by car. The RVL isn't my favorite train line but it sevrves it's purpose. Still time intensive but it's about as close to a middle point you can get.