*In both dramas, there is a power difference between the ML and the FL. *

MAIN PLOT:
-In The Red Sleeve there is love between crown prince (ML) and a court maid (FL).
-In The King's Affection it's the opposite. The FL is disguised as a man and she is also the crown prince who is loved by the royal tutor, who is our ML.

SIMILAR TOPICS OF:
-Childhood nostalgia
-Difficult and abusive family relationships
-Political drama
-Injustice in Joseon; class problems and gender inequality
-Love between people from different classes.
-Multidimensional characters with their own ambitions.
(All of these topics affect the relationship of the FL and ML. )

DIFFERENCES BETWEEN DRAMAS:
-The ML in the Red Sleeve is way more toxic, and the FL is more often helpless despite her clever ways. It can be fun to watch toxic couples, it is like a guilty pleasure but this couple was unusually toxic, in my opinion.
-The King's Affection has a bit more humor. The FL is quite amusing as the 'cold ice prince' while the ML tries to assist her. His love is more pure.
-Overall, The Red Sleeve is more sad and tragic because the FL suffers a lot and she is way more helpless. The FL only wants to be able to make her own decisions. It's realistic, I guess... But it made me cry more and not laugh as much as in The King's Affection, despite this series having a few slow episodes in the beginning.

If you like one of these, chances are that you'll like the other one.