Both of the series are created by Yuu Watase. There is a supernatural side to both of the stories and the main heroines have to put themselves through a test of life to decide what is best for everyone. The main heroines, Aya (Ceres) and Miaka (Fushigi Yugi) have decisions to make affecting the outcome of the people that they love. Each series is also a reverse harem.
These two series are quite different in plot, but having been written by the same author, they have many similarities: similar artwork, similar humour, and a similar love triangle. The overall feel/tone of them is similar, so if you liked one, you'll probably like the other.
Ceres: Celestial Legend and Blood + have twins that have hidden secrets in their past. The heroines of the story do not know what that past is and have to go on a kind of mission or quest to find out how they came to be there. Their dark secrets have to come to light.
In the year 1923, Takiko Okuda is living in a small Japanese town with her dying mother, while struggling with feelings of loneliness and heartbreak. After a long absence, her absentee father finally returns home with a mysterious book named ‘The Universe of the Four Gods,' which he has spent years obsessively translating. When Takiko's mother dies, Takiko vents her frustration and anger on this book her father seems to value above his family; and to make matters worse, she's then transported into the book and finds herself called the Priestess of Genbu. However in this land of Hokkan, the Priestess and her seven celestial warriors are not welcome; Takiko must overcome many trials to save this other world that does not seem to want her.
Although the stories are kind of different, many of the overall ideas are the same; a girl gets caught up in a sea of trouble involving magic and other things that would have been way over her head in the beginning. Also, there is a tug-of-war romance going on between a girl and two guys. Last but not least, the artwork is quite similar.
Years after having been exiled from the Kannagi household for being unable to utilize their fire magic, Kazuma returns to Japan, only to discover that the Kannagi family's very existence is threatened. Unknown to them, during his absence, he had become a Contractor and thus acquired a vast strength in wind magic. Not showing any remaining contempt for them, he offers to help solve the Kannagi household’s problems in return for large sums of money. Together with his younger brother Ren and the feisty heir to the Kannagi household, Ayano, he fights to protect those he cares about... for a price!
With a noticeable tan and bleached hair, Momo Adachi is a high school student who is frequently misunderstood by her classmates. She's accused of being a playgirl and is the subject of many rumors, most of which originate from her ‘best friend' Sae who is jealous of Momo due to them having a crush on the same boy. Momo's life only gets worse when Kairi Okayasu, who she once saved from drowning, decides to make her his own. Kairi coming into Momo's life only encourages Sae to spread more rumors about Momo and Kairi while going after Toji herself, creating quite a complicated situation for everyone involved.
Both series deal with a major love triangle and I had a hard time choosing my favorite guy until the end; each has redeeming qualities about him that makes it hard to choose which one you would really pick. It is interesting to see how much your opinion changes throughout the series when you read it! Both series also deal with high school relationships.