I ride Bolt and Megabus between NYC and Phila quite frequently. They are a great bargain and slightly nicer than the chinatown buses. Either would be fine, but I prefer Bolt for it's higher ceilings and it's Grand Street stop.
Double check the NYC drop off locations as they keep wandering due to the West Side Yards construction.
Correcting above, Bolt is Greyhound owned and Megabus is Coach USA owned.