This is an excellent question...since these two characters are the only women in the play, and they are married to key male characters--Calpurnia to Caesar and Portia to Brutus--it is worth examining.
First, Calpurnia's role is mostly to warn Caesar about going to the the Senate on March 15. She has had terrible dreams and fears for her husband's life...rightly so. However, she fails in her endeavor since Caesar is bullied into going by alternate interpretations of Calpurnia's dream.
Portia, on the other hand, has the role of friend and partner. She gets Brutus to agree to open up to her by slashing her thigh open and challenging him to trust her with the problems he is facing as she can withstand the pain of her wound. He agrees, but the assassination takes place before they have a chance to talk. Brutus is forced to flee after Antony's moving speech, and Portia never has the opportunity to talk with her husband and guide him through his problems.
Ultimately, both wives fail in their goals, but I would vote for Portia as more important since she seems to have a more equal foothold in her marital relationship and would have had a positive effect on Brutus' actions had the timing been right and they had the chance to sit down and discuss matters.
Portia in a dialogue with her husband demands respect from him and declares herself equal to him by the laws of marriage. Because she loves Brutus, she intends to play an active part in his life as his loving wife and partner. Brutus admires her for the things she says, declaring her better than himself. (O ye gods, Render me worthy of this noble wife!)
Calpurnia plays a more passive part. She warns Ceaser but after that she doesn't do much. Ceaser is easily convinced to ignore her which perhaps shows that her opinion is not important to him.
Neither of the two women in this play are able to influence events or save their husbands. However, Portia seems to be more willing to try.