Both are adapted from the Angelique series of games and thus, are both reverse harem. If you like one, you'll probably like the other. However, Neo Angelique is more serious and focused on action while Koisuru is focused on romance and comedy.
Both are vampire romances.
Both are bloody, disgusting, pure gore.
Magical girls.
Both are romance comedies involving women from outer space.
Both are romance shoujo series which are funny but get serious at certain points.