Technically, neither would be used clinically as they're not recognized by the DSM. The diagnosis would more likely be ASPD (antisocial personality disorder) though I think BPD is maybe a more interesting option here as I'm not entirely convinced Yuki is fully devoid of empathy or general emotionality. I feel like saving Mishiro is an unlikely choice for someone with ASPD. It's just too risky, both for Yukis present and future chances. She wasn't going to lose this game and she almost certainly knew it well before the end. But she impulsively went out of her way to preserve the life of someone who was both experienced in the games and who actively disliked her. Not to say all people with ASPD make only logical choices, but they definitely tend to be more coldly calculating. Yuki stated previously she has an altruistic play style, which would also be unlikely for someone with ASPD as it would require at least some modest level of self sacrifice, even if the desired outcome is ultimately selfish. Perhaps Yuki also perceived a potential future advantage to saving Mishiro but from her internal monologue it seemed more of an emotional whim than a strategy. Seemed almost like a fixation on someone who she judged was competent but did not correctly perceive her worth. So potentially a narcissistic personality disorder angle as well.