If I say Aiko I'm pretty sure the whole class will still be confused lol. So pay attention boys.
Aiko is younger I'd say late 30s and I would consider her more attractive and more fit. To the guy now banned, Aiko is sensual and don't know what your experience was but she was really gfe with me.
Yumi is both Korean and Japanese and not just someone claiming to be Japanese. I would say Yumi is in her early 40s, very gfe and I would say a lot more passionate. This girl really gets into it.
It's hard to say so I'll put it as best I can. I'm more attracted to Aiko, but Yumi seemed a lot more into me so whatever points she lost in comparison she made them back up.