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.

Similarity:
=> Our hero who is smart but grew up in poverty "dies" and is reborn/reincarnated again
=> the drama shows the character's early college years where he develops his skills
Difference:
=> Hero is reborn as himself in Again My life while hero in Reborn Rich is reborn as the grandson of the founder of the company he is working for.
=> Hero is a prosecutor and female lead is a chaebol in Again My Life, hero is a chaebol (his reborn version) and female lead is prosecutor in Reborn Rich

=> Jang Hyuk and Jo Jin Woong are in both historical dramas
=> slave characters play important roles in both dramas

=> sageuk set in King Injo's reign
=> action packed kdramas (expect many fights and martial arts scenes) starring actors who are considered sageuk kings and have also played Wang So (Lee Joon Gi of Iljimae played Wang So in Moon lovers while Jang Hyuk did in Shine or Go Crazy)
=> female lead is demure and a beauty (trivia: Iljimae female lead Han Hyo Joo was actually considered for Lee Da Hae's Chuno role).
=> male lead and female lead knew each other in the past, however it takes many episodes before they meet again
=> there is a second female lead who is more boyish and not as demure as the female lead who hangs out and falls in love with male lead (the character has more screentime with male lead than female lead)

=> action packed historical kdramas
=> two rival men and there is a woman loved by both men. one man is connected to the woman's past, the other one is the one she is currently serving/following

=> Both are historical dramas where a mother is the main character
=> They may not have many similarities but both shows have a similar theme: to what extent parents can do for their children. Both dramas depict the good and evil in parents.

Both are fantasy kdramas where actor Lee Tae Ri plays an interesting character in both. Both have bits of Goryeo era in the show.

=> Adult characters reminisce about their youth and first love when they were teenagers in the 1990s
=> There is a sad event in both shows that will make viewers think about the show for weeks
=> Female leads are played by two amazing former child actresses considered rivals by many (Kim Yoo Jung and Kim So Hyun)

Both are historical films featuring intimate scenes between two people of the same gender. The acting is also amazing in both. Song Ji Hyo in Frozen Flower has strong presence.