3 needed to calculate an average
Read one, liked it and want more of the same? If yes, you ought to read the other series.
Both were written by the same person, have a similar style and plot, but different characters. To be exact Naqua-den is a spin-off from Kaibutsu Oujo, so some cameos do appear too. Anyway both series are very alike, though Naqua-den has much cooler male lead.