Both are romantic movies set in a high school that feature love triangles (or more complex shapes in the case of koiboku). Atashi No! is a bit more comedic but both take themselves seriously when needed. Both visually well done in terms of lighting/colour grading/mise en scene. Main male lead is played by a idol boy group member, and Koharu and Mitsuki are played by the same actress <3. Enjoyable acting in both. Highly recommend one if you like the other!
Both movies center two high school students using an "exchange diary" (koukan nikki) and begin with the protagonists having an unrequited love towards their somewhat cold love interest, though the genders are reversed. Both movies also feature some kind of love triangle. The way each movie plays out and ends is quite different, but they both give a similar fluffy, heart-warming feeling. If you enjoyed one you will certainly like the other.
