Soukou Kijo Iris

Both have pretty character designs. Both have a character with pig tails. Both are erotic. Both have very good animated scenes. Both have taboo group sex scenes.  

In both stories some kind of strong-willed woman is taken be force by both humanoid and non-humanoid creatures. As you could guess, both are hentai and extreme one on top of that. 

Both female main character are caught and "attacked" by monsters, orcs, aliens, etc. 

Two girls getting caught by aliens and became pleasure slaves for the rest of their life. 

Both have an excellent budget and released only a month apart from Pixy studios. While the settings are miles apart, with Iris set in the distant future and Suvia set in the dark ages, both have a pretty typical hentai setup with similar themes. Girls get captured, humiliated, and trained to be sex slaves! They also have the monster rape theme going, Iris has aliens and Suvia has ogres and monsters. If you like one of them, you can't go wrong with the other!