Most of the people i know will say its the sub who has the most power because they get to stop a scene, step out from a relatioships whenever something doesnt go as planned. But in my opinion, Both have the same amount of power in a scene/relationship. I say that because both get to stop a scene in the middle if something doesnt go as planned, or If a submissive acts a particular way while a top is doing something(not sure but you can think of ideas) then He/she gets to stop the scene or play date. Same goes for the submissive. Slaves I cant really speak on because I am not a slave and i dont have the mind set as a slave. But I am sure if something doesnt go as planned and their life is in danger, they would stop the scene, the play, etc. It just depends on the person(slave) though, know a few of them who wouldn't stop because they think their "master" or what not could do know wrong but if something goes wrong then all hell has broken lost. Many different agencies getting involved because of simple(sometimes not so simple) mistakes.
So i guess all i am trying to say is, I think both have the right to end a scene, a relationship, etc if the other isnt living up to their part of the deal. Will either one of them take up that right to end the relationship? No one really knows especially when feelings, emotions and that "longing" to serve/dominant, no matter how horrible, good, bad, or indifferent that person(other partner) may be.