Both are magical girl spin-offs starring little sisters from eroge adaptations, with some mecha influences. The major difference lies in that Magical Heart Kokoro-chan is a parody OVA while Nanoha is an actual magical girl series.
"Binan Koukou Chikyuu Bouei-bu LOVE!" / "Cute High Earth Defense Club LOVE!" and "School Days: Magical Heart Kokoro-chan" are both comedic parodies of the magical girl genre, where the protagonists fight against equally ridiculous villains in equally ridiculous premises under the name of justice and love.
Similar sense of humor.Nonsense things happen in even more surreal genre, sentai movie parody. Different is Sunred has it own storyline while Kokoro-chan act as stress reliever after 12 episode of depressing anime.