Similar premise, a simple high school romance which blossoms slowly but heartwarmingly, similar themes of friendship as well.
Similar plots - high school setting, girl falls in love with the class genius, a love rival (severe second lead syndrome here) and the plot progresses to university and beyond
Both are Taiga dramas that focus on the life of a prominent female in the Shogunate - who both have extremely strong and straightforward personalities.
These two shows are drama-spinoffs from their respective novels, written by the same author. Furthermore, both female leads are of the same personality and played by the same actress.
Similar genre, female protagonist. However, Himekawa has a more assertive personality as compared to Sakuragi in Zettai Reido.