definitely Amtrak! not sure if its the cheapest compared to bus prices, but I've done it over ten times to Philly! I book with their app! When I book early it's always super cheap but honestly when I head to Philly or NYC I take the 6 AM train and it's always super cheap haha. Never had a problem, just sleep on the train! Being from Dallas, I'm always super impressed and thankful for this kind of public transportation/infrastructure.