The correct answer is neither. The sentence "We shall step upon white down" is neither a simile nor a metaphor.
Similes and metaphors are figures of speech mainly used in literature to provide a special effect to a piece of writing. Both similes and metaphors are based on comparisons; however, there are differences between them. A simile compares things using the words "like" or "as". An example of a simile is: "She was as white as snow". On the other hand, a metaphor states a comparison in a direct way using the verb "be". "Her eyes are a poem" is an example of metaphor. In that way, the sentence "We shall step upon white down" does not establish a simile nor a metaphor because it does not include a comparison between two things.