As much as I love the SUPERTRAIN, Amtrak is a wee bit pricey, especially if I fail to get 14-day advance tickets. There is a lot of competition between bus carriers, and last time I went to DC it was $20 roundtrip which is pretty cheap for a 140 mile (each way) trip.
Amtrak is significantly faster, specially between Philadelphia and DC, and much more comfortable, but the bus is ok.
