Of all the options listed, a Venn diagram and a compare-contrast chart would help the most with comparing two topics.
A numbered list isn't structured to compare topics, and is meant more for laying out information and making it easier to understand.
A cause/effect graphic organizer wouldn't necessarily be helpful to prepare for writing the essay, unless the topics had a correlated effect on each other (the cause/effect organizer would only work in certain situations, like comparing events).
