One approach that you can take for this comparison/contrast essay is by first researching the basic tenets of each writer. For instance, as a Transcendentalist, Emerson held many of the views of the Romantic tradition, such as the idea of communion with nature as a symbol of the spirit of man. Also, Transcendentalists believed that God is present in every aspect of Nature, including every human being. A corollary of these beliefs is an optimistic view of the world as good and evil as nonexistent.
On the other hand, while Poe, too, is part of the Romantic movement, he is associated with what is known as the Dark Romantics. He acknowledged the existence of sin, pain, and evil in human life and formed a counterpoint to the optimism of the Transcendentalists. And, while there is a consideration of the natural world as a means of discovering the truth that lies behind mere reality, this truth is often malevolent, and not always good as the Transcendentalists believe. Nevertheless, both Emerson and Poe share the idea that intuition is superior to reason. (Prince Prospero, in his arrogance, reasons that he can defeat the Red Death since he has fortified his guests and himself from this evil; however, he is defeated by evil spirit of death.)
After you record the basic tenets of each writer, peruse the literary pieces again, writing down passages which exemplify the tenets they have in common, and in which tenents they differ. (The use of a Venn diagram works for this task.) Then, when you have finished, you may wish to review the link cited below which offers step-by-step assitance on how to write a comparison/contrast essay.