Portia is initially apprehensive to the point of being melancholy about the possibility that Bassanio will make the wrong choice. This brooding quality and penchant for melancholy is characteristic of a romantic hero or heroine. However, she is also filled with the potential hope that Bassanio will choose correctly. Hope in the face of possible hopelessness is also a Romantic theme, namely with Keats, but many other Romantic poets as well.
Romantic themes include the focus on the individual, intuition, a connection with nature (in the sense of the natural world and/or having a spiritual or transcendent connection with nature). Logically following, a Romantic heroine also tends to have the characteristic of "transcending" social laws and strictures in pursuit of some goal or quest. Portia's romantic wish is to be with Bassanio. She overcomes her brooding melancholy and decides to trust her intuition (and with faith in Bassanio's intuition and logic) that Bassanio will choose the lead. After Bassanio chooses correctly, Portia sets out on a quest to help save Bassanio's friend, Antonio. At this point, she becomes one of, if not THE, main protagonist, the heroine who inevitably saves the day.
In these lines particularly, Portia tries to imbue the act of choosing with poetry, comparing the potential choices to music. Doing so, she hopes to give Bassanio a more transcendent, romantic description of her love. After her description, she says, "Now he goes, with no less presence but with much more love." She tries to inspire the same sort of Romantic intuition that she has onto Bassanio - so that he will make the right choice. Bassanio must make a choice regardless of the social or monetary value of the gold, silver, or lead. Like a Romantic hero, he must make a solitary choice from his own intuition.