The former is the smartest out of the quintuplets, while the latter is the least intelligent.
The special store illustrations done by Negi Haruba to promote Volume 10 of the manga also depict them as a band with the same instruments as typically assigned to these character roles, save for Miku being a bassist and Itsuki being a keybordist with castanets.
She does eventually cave to being tutored like the rest of the sisters.