AA2 offers older graphism but with decent and numerous sex scenes and animations and more gameplay :
- You can play male or female, student or teacher.
- You can swap the main character at begin of the day.
- You can fight, invites peoples in activity/fun etc...
- NTR (You may have to compete for your love one(s) with other student, you can cheat with other people too)
I strongly recommend using AA2Mini.
Koikatsu has more modern graphism, more way to customize your Waifu but less sex animation and less gameplay. Your are the only dude of the school.