If you like one, you'll most definitely like the other.

Similarities:
- Main leads are childhood best friends
- ML has liked the FL for a long time but is too scared to admit it
- FL has trouble in her career, so the ML helps her
- Close proximity helps develop their relationship
- Love triangle (2 guys, 1 girl)

Differences:
- LND focuses a lot more on family relationships as well as romantic ones and has a more full plot with side characters that have their own story (like the main character's parents, the other bsf, and her brother) because it has 16 episodes whereas S#1 only has 4
- Soundtrack is more of a fluffy romance without much drama, but LND has a lot more plot twists and other factors to their relationships that cause a lot of delay

I personally loved both of them, but I will say that if you like shorter plots that get to the point faster and don't run you through a mix of emotions, then you should watch Soundtrack. But if you like seeing a secondary couple and also seeing the main lead's family and friends and seeing a more realistic approach to romance, then LND is your best fit. Hope this helps!