Sera is a character that, unlike Cecille, is pure both in body and in mind. Cecille was always aware of her body and used it to manipulate others in her environment, but Sera uses her body to fight and defend. She takes action and physically exerts herself in order to provide safety for the hero. This is a stark contrast to Cecille, who at first hides behind the hero and starts manipulating him the day they meet.
Beyond that, Sera is also very strong in mind. She actively fights the curse, unlike Cecille who only pretends to do so. Sera's feelings and thoughts are always honest and genuine, unlike Cecille who always lies to everyone around her.
So naturally, humiliating and degrading Sera is more perverse and empowering (edit: I would like to add that in order to reach this conclusion, you first have to play the game and realise that Cecille is lying to herself. I like both characters!). Influencing her enough to distract the hero while he sits next to Cecille while you fool around with her is actually a crucial point in how she behaves. I do believe this is also why that part of the game got a mini-game (and it should). It is as good as a corruption scene can get. You not only change the behaviour of one, but two heroines at the same time. Delightful stuff.
And then you get Cecille x Sera, where you torment Sera and witness Cecille's perversion come to light. There is power-play, there is corruption, there is some lesbian action, what more could you ask for?
The culmination of Sera's story is where you start fighting her, and I think that is also a very different approach to Cecille. With Cecille, the threat of violence gets her off, not the violence itself, but with Sera, you have to actively hit her to get results. I know this part might seem misogynistic, but sex has elements of violence, and while no woman I have been with would have enjoyed me punching their stomachs, all of them enjoyed it immensely when I choked them. So I can get behind beating the shit out of Sera simply because in this setting, she can take it, and the violence is a conduit to pleasure.