There are strong differences, of course. Spike just wants Buffy to love him. In many ways, he follows her lead, but his lack of soul makes it difficult for him to gauge when he’s abusing her and when he isn’t. He’s an emotional predator, and we see that on the balcony in Dead Things. She’s ashamed of what they do together, so he uses it to isolate her from her friends, to convince her that she is bad and dirty and has no choice but to be with him. He deliberately crushes her happiness and exacerbates her feelings and self-loathing. He’s impulsive about it - only a few episodes later he is touched to see her happy. But that makes what he does no less real.
His encouragement of the alley beating is real, too. Buffy’s a big girl and Spike doesn’t force her to do anything, but he encourages her. He lets her. He wants her to beat him, because the more horrible she is, the more ashamed she feels, and the more (from his perspective at the time) she is under his power.
Ruby comes at it more deliberately. She doesn't want love. She wants control over Sam in order to achieve her goals. While she may have enjoyed having sex with Sam, I think she initiates sex in order to have power. Dean’s “banging monsters” comment in Sex and Violence underlines Sam’s very real and very legit reasons for being ashamed of having sex with Ruby, for not wanting Dean - or Bobby, for that matter - to know.
While Spike wants Buffy’s friends to know the truth, Ruby initiates the lie to Dean, in such a way that continuing the lie is the path of least resistance to Sam. Later, she encourages Sam to tell Dean the truth, but she’s already set it up. They’re co-conspirators. Telling Dean the truth would be admitting that minutes after Dean walked into the door from Hell Sam lied to his face about something pretty significant. In that moment, when Ruby wondered if Dean was Sam’s lover, Sam chose Ruby. “He’s my brother,” he said, and in some ways, he really did choose a demon over his own brother.
This entry was originally posted at http://itsnotmymind.dreamwidth.org/64964.html. Please comment either here or there, whichever works best for you.