Both dramas deal about 2 women that are divorced and go through a hard time. After a long jumping between jobs, both women end up, with the help, from in Romance is a Bonus Book, a friend, and in Cunning Single Lady, ex-husband, at a company as intern. Both have that man as their superior and keep it a secret from their coworkers.
Both stories give off such a similar vibe. Both women reminded me a lot of the other while watching the dramas.

Just one thing: please ignore the horrible synopsis of "Cunning single lady", it gives such a very bad impression, and trust me, it is completely wrong. There is by far more to the drama than what one thinks at first.

Both deal about friends who realize they feel more than friendship for the other person. Both deal about the nearly same story, so if you like one of them you are bound to like the other as well

Both dramas deal with mothers who's daughter was murdered and look for the truth of what happened.
Both mothers are strong women.

Both deal about a small group of people that receive orders from a *leader* to investigate cases. Both give off a similar vibe. Both deal with characters that have a tragic past and try to get revenge.

Both kdramas deal about childhood friends that love each other. Both deal with a life-changing condition. However "More than Blue" has a much more tragic take on this and is a movie, whereas "I need Romance 2" is dealing with this topic in a more fun and lighter way

Both deal about childhood friends that love each other. In both dramas Lee Jin Hook plays the male lead, the pair is 30+, lives next to each other and tries to solve their problems with their feelings and the extra guys/girls of their friend.
Main difference: I need Romance 2 the pair already dated.

Both mystery dramas deal with objects that allow the protagonists to "time travel". In both dramas their is a main couple that is somehow intertwined with each ever time.

Both dramas deal about prosecutors, family murder and in both dramas the main leads try to find out what truly happened that lead them to be in the current situation.

Both are historical dramas that deal with a 2nd born son who tries to avenge and find his family. Both have a fun and strong (although in Princess Man stronger) love interest.

Both movies are about families that have to deal with the aftermath of a brutal crime that has been done to their daughters.
Both fathers show the extent of what they are willing to do for their daughter.
Both are brutal, tear jerking movies.

Both movies have the same actress for the female Lead, both deal about a romance story where one of the pair has to deal with a disability/condition.
Always is more dramatic than The Beauty Inside.

both deal with ghosts, both have a similar atmosphere, both have a fun humor