Both anime are about demon's that have to finish off what they started and get into a relationship in some form. Hitomi kara Destiny is shoujo and short where Demon Diary is shounen-ai and more fleshed out because it is longer, but if you enjoyed one you'll like the other as well.
Ishuca is a man who has been told he is going to die. So as to not disturb anyone, he travels to a remote cave where he encounters an ancient demon known as Blood; trapped in the ice for centuries, Blood is about to eat Ishuca when the man offers his body to the demon. While Ishuca then dies naturally, Blood is so touched by the man's kindness that he inadvertently revives him with his tears. Together, the two set off on a grand adventure and unintentionally try to breach the rift between humans and demons; for they are not as different as they may seem to be.
Both extremely cute centering around the love of two male characters. Both are fantasies with bunches of humor and "OmiGod That's so cute!" moments abound. Both main characters fall in love with a demon, but the plots are entirely different, so it's not like reading the exact same story either.