=> Both are kdramas of the same story: My Dearest. Each of the two shows recommended are two seasons of the same show.

Both sageuks have really good plots with the same writer. Both male and female lead are strong characters who have to overcome a lot. Both have good OSTs, Main difference is that there is less focus on romance on Rebel Thief than My Dearest and that the main couple in Rebel Thief are friends for long before becoming a couple. Rebel Thief also has a few fantastical elements not present in My Dearest.

Both sageuks have really good plots with the same writer. Both male and female lead are strong characters who have to overcome a lot. Both have good OSTs, Main difference is that there is less focus on romance on Rebel Thief than My Dearest and that the main couple in Rebel Thief are friends for long before becoming a couple. Rebel Thief also has a few fantastical elements not present in My Dearest.

=> Both are historical Kdramas partly inspired by Western love stories. If My Dearest is partly inspired by Gone with the Wind, (as described in wikipedia), The Princess' Man is partly inspired by Romeo and Juliet.
=> Earlier episodes contain fun and lighthearted scenes before political events changed everything
=> Female lead rides a swing in both kdramas, both scenes contributed to male lead falling in love with her
=> There are scenes in both dramas of couples trying to run away.
PS: ForThe Princess's Man fans, the show being recommended is Part 1 of My Dearest, you should check part 2 for the continuation of the story.

=> Both are set in the Qing invasion of Joseon
=> early parts of the story revolves around marriage but slowly turns to war and carnage
=> Both dramas depict how the Qing invasion really affected many people but young people who are at the stage of their lives to fall in love and get married have quite a focus.
=> There are important Qing characters.
=> Strong male and female characters (my fave actress Moon Chae Won got many awards for her depiction of the female lead in War of the Arrows => people should watch her sageuk works especially this, Princess Man, and Painter of the Wind).
PS: For War of the Arrows fans, the show being recommended is Part 1 of My Dearest, you should check part 2 for the continuation of the story.

While the historical events in both dramas are not super accurate, you can watch both to see a glimpse of Korea's history from the time of the last Joseon king to the time of the Japanese colonial period. It is best to watch Mr. Sunshine (covering last Joseon king's time period) before Song of the Bandits (covering Japanese colonial period).

=> Both are historical dramas set in the Japanese colonial period of Korean history. The two main male leads are former friends and they love the same woman. However they became estranged as one became dedicated to the cause of Korea's freedom while the other became loyal to Japan.

Both are sageuk by the same writers. Shin Se Kyung is in both dramas too. Queen Seon Deok is her first drama with this writing team. If you see both dramas, you'll basically see how Shin Se Kyung is as a teen actress (playing younger part of one of the main characters) in sageuk versus as a more mature lead actress. Another similarity is that both Arthdal Chronicles and Queen Seon Deok have twins as main characters and there is a prophecy regarding them in the shows.
For Arthdal fans, you might want to check Queen Seon Deok for Shin Se kyung's acting as a child actress.

Both are sageuk by the same writers. Shin Se Kyung is in both dramas too plus season1 Eunseom/Saya Song Joong Ki.
For Arthdal fans, you might want to check Tree with Deep Roots for Shin Se Kyung and Song Joong Ki's acting .

Both are early 2000s wuxia movies starring Zhang Ziyi. She was only a newcomer when these movies were made.

=> 3 male leads and one female lead sageuk drama
=> According to Secret Romantic Guesthouse's plot summary, the four friends will work together. It was basically the plot of Sunkgyunkwan Scandal in the latter episodes.
=> The male leads in Secret Romantic Guesthouse and all the main characters in Sungkyunkwn Scandals are young scholars.

=> Male lead is very rich but both started out very poor in the story (in reborn rich, male lead was poor in his original life then became rich in his reincarnated life) while female lead is a prosecutor/former prosecutor.
=> The world of money, business, corrupt officials and business people somehow affecting the elections
=> Prosecutors are featured as important characters in the show
=> You need to understand some legal, government, and business jargon as well as the first or first two episodes to understand what is going in both both shows.