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

Both dramas deal with powerful plotting women and men that are changing sides. In both dramas a murderer plays an important role and involves nearly all the people.
Although the K2 has more action to offer for now, the suspense and the mystery and plotting between the characters is a lot similar.

Both deal with police officers who talk with people who are involved in a crime/or about to commit a crime.
Both have a main pair that tries to solve one big case that connects them via losses

Both dramas have a similar with how they portray those "time skips". Both deal with a detective who has a connection to the future. In Signal via a transponder, with Tunnel via a Tunnel. Both are crime/investigation kdramas.
Both male leads have had once a love interest who was played by the same actress.

Both movies focus one a criminal who reaches out via a live broadcast. Both movies have a live debate with the culprit. However Terror Live is more intense.