Faith is my favorite Korean drama featuring time travel. These two dramas share similar concept of time travel, which a modern man in Mr Queen, a modern woman in Faith travelled back to the historical era.
So, I recommend Shin Don only because it has common historic characters with Faith. You'd watch it if you have educational purposes (want to learn more about the period).
If you take away the supernatural time-traveling theme with the main girl, it revolves around King Gongmin, Princess Nagook and General Choi Young who seem to play a bit role in the background of the Six Flying Dragons. It's definitely a good drama to watch either before or at the same time as Six Flying Dragons to get a good background picture as to why Goryeo is looking the way it does in Six Flying Dragons.