from jianguomen to badaling great wall. is it better to get the bus or the train?

MP, it costs CNY6 by train but CNY12 by bus if you don't have Beijing Smart Transportation Card. If you have the card, the bus ticket should be CNY4.8.

I personally think you may take the train to visit the wall. You just need to take subway line 2 from Jianguomen to Xizhimen, get out from Exit A1 and then walk to Beijing North Railway Station. If you have the transportation card, you can swipe it to board the S2 train. Otherwise, you need to queue up for buying the ticket. One more thing you should know is that passengers don't take their seats according to the seat number. They just take seats randomly.

For your return trip, you may take the train too. I went there two times. The buses are crowded, especially when you get down between 15:00 to 16:00. You can take S2 train departing at 08:34. It arrives at 09:47. Then you may spend two to three hours on the wall. At this time, you can choose to take the train or bus. There won't be too many passengers.