Though both women are wives of noble men, the way they are treated by their husband (and the way they treat their husbands) is quite different, as depicted by Shakespeare.
Portia is depicted a strong-willed and stubborn. She knows there is something troubling her husband and is relentless in her pursuit to find out what is it. She uses different persuasion techniques to prove that despite the fact she is a woman, she is strong enough to be placed in his confidence. Portia sees herself as an equal to Brutus--someone Brutus chose to be his equal--rather than the traditional subservient role as a wife. Later, Porita plays into the stereotype of the hysterical woman. Brutus has obviously told her the assassination plan. She is frantic about what is happening at the Capitol, even mentioning that her woman's heart is weak.
Calphurnia, on the other hand, is shown as subservient to her husband from the beginning. She refers to Caesar as "my Lord", and does what he commands her to do (stand in Antony's way so he can touch her and cure her of her infertility during the foot races). When she has foreboding dreams about Caesar's death, she begs him not to go to the Capitol. Rather than giving in to her because she is his wife and his equal, Caesar treats her like a child who's had a bad dream--he is condescending and agrees to stay home to make her happy. He is quick to change his mind, however, when he is convinced it will make him look weak to submit to his wife's wishes.
Katemschultz said it quite well but he forgot one major part. As Portia kneels to persuade her husband, Brutus says, "Kneel not, gentle Portia." Portia is more of a modern woman who quickly retorts, "I should not need if you were gentle Brutus.
While on the other hand, Calphurnia kneels to persuade Caesar and he takes no note to let her stand equal with him.