Hostility between Scooby friends and shadow-selves definitely seems to be affected by gender. There are a lot of parallels between Spike and Xander (they both have unrequited crushes on Buffy that they are initially entitled about, and both attempt to rape her while under some supernatural influence), and they can’t stand each other. Willow hates Faith, and the two are almost direct opposites in S3: The very unsexual goody-goody and the hyper-sexual “bad girl” (And may I just note how not thrilled I am by the correlation between sexuality, especially female sexuality, and "badness"?). Buffy’s relationships with her Scooby friends have a pleasant surface that hides a lot of suppressed resentment and anger. Her relationships with her shadow-shelves are combative and violent, but underneath the hostility is a deep, instinctive bond.
Buffy is an outsider with her Scooby friends. She’s the “hero”, the “Slayer”, the weird one. For better or for worse, the special one. In “Out of Sight, Out of Mind”, she feels left out when Willow and Xander reminisce about their elementary school days. In “When She Was Bad”, she dreams of Giles strangling her while her friends watch on, unfazed. In “Becoming, Part 2”, she feels so estranged from her friends that she leaves town without even telling them.
She connects with her shadow-selves, but she doesn’t belong with them, either. She has responsibilities that they couldn’t understand. She never entirely gives up on her other relationships, leaving Spike and Faith, who have no one else, feeling left out. They belong entirely to her, but she will never give herself entirely to them. “I am not alone,” Buffy tells the First Slayer in “Restless”. Faith and Spike are, and they ache to be Buffy. To have friends, to have a family. “There's trees in the desert since you moved out,” Buffy tells the First Slayer. “And I don't sleep on a bed of bones. Now give me back my friends.”