Angel's Feather

Earthian and Angel's Feather are remarkably similar in almost all points. Both follow the same general plot of two opposing factions of angels fighting for supremacy. More, these two OVAs involve shounen-ai overtones (perhaps more pronounced in Earthian but very clear in AF as well) that make for some of the more dramatic moments. Earthian has a more dated look while AF is visually much more modern; seen side by side these two anime provide an interesting comparison.  

Despite the fact that Angel's Feather deals with supernatural while Marginal Prince doesn't, they both take place at an academy, explore boys' relations and have similar character types. 

Both are shounen-ai and take place in an academy. The boys only kiss; they dont have sex. 

Both male leads can't remember another boy from their past. Also, both are BL, and made from erogies(hentai games). 

Angelic bishounen involved in a conflict is the basic idea behind both Angel Sanctuary and Angel's Feather. Both are somewhat disjointed due to fact that the original formats of manga (in the case of AS) and game (in the case of case) contained too much information to be fully explored in these OVAs. A dramatic romantic involvement that includes incest (much more obvious in AS but heavily hinted in AF as well) and a supernatural element that relies very heavily on wings/angel-related imagery make these two anime a perfect match.  

Loveless and Angel's Feather take a dramatic approach to shounen-ai mixed with magic. Both Angel's Feather and Loveless include supernatural battles and a growing romantic relationship between male characters. A very vibrant and elegant design pervades these anime as does the occasional feeling of fragmentation that makes one feel that the whole story is about to begin exactly when it comes to an end.