First, there are quite a few similarities between Bassanio's relationship with his friend and his relationship with his wife. He depends upon both of them financially; both are apparently wiser than Bassanio, and they both show mercy upon him (Antonio when he lends him money in the first place, even though Bassanio already owes him money, and Portia when she forgives him of giving her ring away).
The difference lies in how the two characters influence Bassanio. Antonio could have, as a mentor to the young man, given him better suggestions on how to win the fair Portia's hand rather than adding to his obvious spending problems by lending him more money. In contrast, Portia does not let Bassanio take the easy way out of his troubles. She is more proactive. For example, when Bassanio travels to Venice to try to save his friend Antonio, Portia immediately recognizes the need the follow him and address the situation herself.