In what ways are Isabel and America different?

This is an interesting question. Most of the time, Isabel and America are compared to each other instead of contrasted from each other.
A simple difference is that America is a country and Isabel is a person, but that is an incredibly simple contrast and not likely what your teacher wants to hear.
One difference is that Isabel starts the story as a free slave, but America is not free from British rule. The country hasn't declared independence yet. The story begins in May of 1776. While America might be getting ready to declare independence from Britain, it has not happened yet; however, Isabel begins the story having recently been set free. She is forced back into slavery, and soon after, America becomes its own free country. This is a sort of role reversal that occurs in the story.
I would say that another key difference is that Isabel is willing to work with the British in order to become free again. That is why she tries to escape the Lockton household by going to the British military. Isabel has heard that the British are willing to free slaves that help them. Seeking help from the British is not something that the newly formed American country can do.