The similarity is that both animations are base on the romance of three kindoms, and the main charaters are females. They reinterpret the original novel. Both of them contain some erotic scenes.
The difference is that Ikkitousen is more violent and Koihime Musou(恋姫†無双, Koihime†Musō) is more humorous.
Ikkitousen tells the story from the view point of Wu(吳). One of the most important protagonist is Sonsaku Hakufu(孫策 伯符, Sunce Bofu). On the other hand, Koihime Musou follows the viewpoint of the original novel. Kan'u Unchō(関羽 雲長, Guanyu Yunchang) of Shoku(蜀) is a main protagonist.
The backdrop is also different. The backdrop of Ikkitousen is Japanese schools at present. The charaters are students. However, the setting of Koihime Musou is the ancient China. That does not mean Ikkitousen has no linkage with the three kingdoms. Most of the main charaters have some spirit from the ancient China.
Ikkitousen is a school-action animation. From the first scene, the students keep fighting. There is no sufficient reason. Sometimes, they fight just for fight itself. By contrast, Koihime Musou is a fusion historical animation. If you catch a glimpse of it, you may think it's kind of historical animation. But sooner or later you can realize that it ignored the history.
