Answer:
They differ by their mood or theme.
Explanation:
Both poems "The Cremation of Sam McGee" and "Little Boy Blue" are about the death but their situation are different. In the poem "The Cremation of Sam McGee" first has a mood of  frustration but change to sadness when the story goes forward while "Little Boy Blue" has a sentimental but beloved theme. So both poems have different theme or mood from one another.