The main characters are very alike - misunderstood, and they hold another side to them. The main male characters are also very lovable. The plots hold this feeling of sweet innocence of young love. If you like stories with cute storylines, lots of fluff, and good art, both of these manga are for you.
These two are similar because they are both manga's about the exchange of letters, and the love. Though, these are not exactly the same, nor same ending, it's still a really nice story, I think anyone would enjoy. Also, it's a one-shot!
Both series are incredibly short, being one shots, and both deal with awkward romance. System of Romance deals with a girl realizing she had fallen in love with a dangerous felon, and Haikei, Nanashi-sama dealing with a girl sending sweet messages to an anonymous admirer. Both series deal with odd, round-about, but somewhat sweet methods to falling in love.