I think it's just that Minthara is hotter than Shadowheart. She is more conventionally feminine in appearance, at least.
Strange, I feel like the polar opposite, but maybe that's because Shadowhearts personality is much more feminine. And that's actually the reason I like Minthara more, not a fan of effeminate females.
I think Minthara is feminine, which is great. Femininity is not necessary should be an archetype "Damsel in distress"
She can be sentimental and sweet in certain moments, strong and confident in some moments, cruel sadistic in some, but at the same time remains feminine. Masculinity is Cassandra of the Dragon Age of the Inquisition, a hideous character. Minthara is fine with that.