Obviously having a twin harem is the only correct option, but if you had to choose a secretly favourite twin of the two, who would it be?
Not trying to be divisive here, just state what each twin does/has/is that you like.

Personally, I put Fuwawa a bit ahead of Mococo. I like Mococo's energetic, heart-on-her-sleeve personality, but I love the contrasted Fuwawa's calmer demeanor (except when violence, which doubles as gap moe). Also her tits are enticing.