Depends on who you ask and what their overall opinion of relationships is.
A weak woman will say the man has the upper hand because he is the "breadwinner" and makes the rules to dictate how life will be in the household. She defers to him in all matters and does not think for herself.
A strong woman will say that neither person has the upper hand in a GOOD relationship (one of mutual love, respect, and honesty) and that both people share equally in any matters of "power" that arise.
A woman who uses her body to manipulate weak men into giving her what she wants will say that she has the upper hand.
A weak man will say the woman has the upper hand because she bosses him around and nags him into doing what needs to be done or what she wants done.
A strong man with control issues and well-hidden insecurities will say that he has the upper hand because women should know their place and stay there.
And it could go on and on...
...women have the upper hand because they have a vagina.
...men have the upper hand because they have a penis.
...women have the upper hand because they can call the cops at any time and lie to get the man in trouble and the cops will believe them.
...men have the upper hand because they are physically dominant and can intimidate a woman into being subservient.
...blah, blah, blah.