I don't believe either of their marriages are happy, but I'd say Brutus and Portia's is a little stronger based on them having more mutual respect for each other. In both cases the woman seems to care more about the man. Calphurnia shows a great deal of concern for Caesar when she dreams of Caesar's death on the Ides of March and pleads with him to stay at home. Caesar shows a little respect for her for intially listening to her pleas, but ultimately dismissing her when Decius Brutus contradicts Calphurnia. Caesar, on the other hand, shows some resentment for Calphurnia when he tells Antony in Act I, Scene II, "Forget not in your speed, Antonius, to touch Calphurnia, for our elders say the barren, touched in this holy chase, shake off their sterile curse." This shows Caeser is unahppy with Calphurnia for not being able to bear him a child. He shows a lack of respect when he announces her sterility to Antony and those nearby. The publicizing of this private condition is surely an embarrassment to Calphurnia.
As for Brutus and Portia, Portia shows genuine concern for Brutus when his behavior becomes erratic. She wants Brutus to share his problems with her and hopes she can help him through them. Brutus is reluctant to tell her, but eventually promises he will confide in her. Though it is not in the play itself, the reader is led to believe Brutus was true to his word because later in the book because when the assassination is about to occur, Portia is worried sick about Brutus and is seen running through the streets in despair. Brutus, however, doesn't show eqaul affection for her. Portia ends up killing herself by "swallowing fire" due to her worries that Antony and Octavious might kill Brutus. In Act IV, scene 3, when he reveals the tragedy of Portia's death to Cassius, he responds in the most stoic of ways, "Speak no more of her. -Give me a bowl of wine - In this I bury all unkindness, Cassius" (lines 161-162).
Both relationships are one-sided, yet it appears Brutus treats his wife like more of an equal than Caesar.