The first pair in the lifestyle I met, and who introduced me into the lifestyle, was kind enough to answer my excessive amount of questions with lots of patience and understanding.

The dom told me that a dom has only as much power as the submissive will let him have.

As denu and others pointed out, the submissive can at any point leave the relationship. So basically, the power to end it is equal.

That said, the power within a scene is in my eyes different. As in denuīs nicely put example - in certain scenes, positions, bindings, gaggings etc. the submissive has really no control, meaning she has given that control completely over to the dom.
It is up to the dom to handle that situation in a way that is good for both.

I am with hubby almost 12 years now. Although I personally have issues with handing over control in real life issues, I give over control within a scene. Although this is sometimes extremely scary for me, it turned out to be the biggest turn-on for me at the same time.

As for the power within the relationship: I found, as I learned over time, to ask for his opinion on things, simply because there are a lot of things he knows more about than I do, and because he is usually right (yay, I admitted it! *lol*).

I donīt think the question can be answered for all couples in a one-off sort of way.

Basically, I could end a scene at any time, by giving my safeword.
While I have considered it sometimes, I never did that yet.

Basically, I could end the relationship at any time.
The thing is, I donīt want to do that.

I donīt know if that helped you. Itīs just my opinion.

Kind regards
Arria