Which detail is most important to include in a summary
of the excerpt?

i would say the correct answer is not concrete.

a compare and contrast essay is a piece of formal writing in which the author focus on the similarities between two ideas, objects, places, etc., and the differences between them. there are some aspects that the author should take into account when he/she is crafting the essay. first, the author has a clear idea of the effect that he/she wants to produce on the audience (a purpose). his/her main idea should be presented in the thesis statement in a simple but effective way which gives the audience a clear idea of the content and the direction of the essay. in addition, it is also important to focus to the similarities and differences that not only are interesting and relevant to the reader but also provide a new, impressive point of view or deeper understanding of a topic.