I wouldn't recommend the Chinatown buses for a couple reasons. Traffic is one of them. Plus, a news station recently did an investigative piece and it turned out that a lot of the drivers weren't licensed. They also showed that the buses break down routinely.

SEPTA/NJ Transit is the way to go. R7 Trenton line is what you want. I believe the fare is $9/RT on SEPTA and the NJ Transit line is quoted above. Transfering over at Trenton is very easy and the station is small. Also, alot of people do this so just ask if you have any questions. Keep in mind that if you are starting out at 30th Street Station in PHL that the Regional Rail lines (aka: SEPTA lines are in the far back corner- not in the great hall where the Amtrak lines are).

Also stated above, if money is no object, then go with Amtrak (and collect some points...). It's much easier and a little quicker with less stops.

I use SEPTA/NJ Transite for personal trips and Amtrak for business trips.