Yeah they are both far from ideal but I think Miyabi would be much more healthier for him. She is just a classic tsundere who genuinely likes him and had to be seperated from him because of her mother's pressure while Hizumi is an enabler red-flag. Altough my mind can change according to...
It is unrealistic even for this standarts. High level chess heavily requires pattern recognition and game knowledge, what she did was basically superhuman level