I really feel like Medusa and Marie are Stein’s bad and good sides. Marie represents stability for him and her motherly attitude towards the students is something that reflects on sane Stein in the manga. Medusa, however, holds his same sadistic attitude and tends to act blindly without worrying of consequences like madness Stein. And when stein completely fell off the deep end it was because he thought Marie was dead and fled to Medusa if i remember correctly. She also has his desire to learn at any cost. Personally, I prefer Stein x Marie.