"Aesthetically pleasing" is more than just beauty, I think.
Sayu is gorgeous, sure, but she has a more striking beauty and her beauty is prone to breaking in certain circumstances. Off the top of my head, her face sometimes swells up (she says so herself) and she tends to snort-huff when she laughs. See also Maimi and her toothy, squinty grin.
Mizuki, on the other hand, isn't quite as strikingly beautiful as Sayu or Maimi, but her beauty is more consistent and soothing. Just looking at her makes you go "Ahhhhhhh," as if you can't imagine it being any other way. That's the golden ratio.