First of all, the similarities:
Portia and Nerissa are close friends and confidantes as are their male counterparts. Both Nerissa and Antonio go to extreme lengths to aid their friends. Nerissa is willing to cross-dress and deceive the Courts without knowing the reason. When she questions Portia, she is told: "I'll tell thee all my whole device...When I am in my coach..." (3.5.81-82). Antonio is willing to barter a pound of flesh with the Jew in a loan for Bassanio, so he can attempt to win Portia's hand: "Yes, Shylock, I will seal unto this bond" (1.3.172). One character in each of the couples (Portia and Antonio, respectively) has power and wealth.
However, it is here the similarities appear to end. Nerissa is a friend but also Portia's waiting-gentlewoman, and she becomes more of an equal in the romantic plot as both Nerissa and Portia are involved in the ring exchange. Nerissa is a co-conspirator and friend. On the other hand, Antonio appears to aid Bassanio out of his own self-interest. In fact, his love for Antonio is so great that upon, what he believes are his last moments of life, he proclaims: "Say how I lov'd you...bid her be the judge...Whether Bassanio had not once a love" (4.1.276-278). Bassanio replies,"But life itself,my wife ...Are not with me esteem'd above thy life" (4.1.285-286). Hmmm. I would suggest that Antonio's relationship with Bassanio is much more complicated than the one between Portia and Nerissa.