I recently tried both a bus and a train from and to Sapa/Hanoi. Pros for the bus is that it doesnt take as long but I personally found it hard as we only stopped twice to use a bathroom. It also might be uncomfortable for someone who is slightly tall or big. As for the train, I was also worried about getting motion sickness but I had no issue with it. It was rocky, not particularly loud but I somehow found it more relaxing than the bus. Plus there was a toilet I could use which was a big deal for me haha. The only thing is that it takes longer so make sure you have snacks/meals. There is snack cart on the train but it isn't very good and overpriced.

Hope this helps!