Assuming all other things equal (price for flights, mileage credits, flexibility of travel), PHL is closer to Trenton and is only one highway away. I 95 (not the NJTpke) connects PHL to Lawrenceville.
To go to EWR, what are you going to do: Take Route 1 north to NJTpke? Take 206 North to 287 North to 78 East?