What are some things I can compare in "A Letter to the World" by Emily Dickinson and "A Man Said to the Universe" by Stephen Crane?
You could compare the poetic rhythms of the two poems. Dickinson's poem for example uses the same rhythm and meter that all of her poems use. Crane's poem, on the other hand, is free verse.
You could also compare the importance of nature to both speakers. In "A Man Said to the Universe" there is an implied importance to nature and the universe for the speaker. If the speaker didn't care at all about nature, then he wouldn't have a need to express his existence to the universe. Dickinson also places an emphasis on nature. To her, nature is regal and tender and a source of knowledge, which is a huge difference from Crane's poem. In his poem, nature/creation is coldly indifferent to the presence of the speaker.