To begin, I would suggest that you ignore the general context in which we typically think about the Star Spangled Banner. The Star Spangled Banner was never meant to be a song - it was originally published as a poem. After many years, the first stanza of the poem was made into the United States National Anthem.
Next, it might be helpful to do a little research into the authors of each poem. Where and when the poems were written? What are the author's other poems like? What are some similarities and differences in their biographies and styles?
Examining the poems themselves, each describes a battle, either literal or metaphorical. The narrators of each poem have a very distinct reaction to the battles they are witnessing and describe them in different ways. On a very basic level, you can compare and contrast use of figurative language.
Having briefly reread these poems, for example, Key writes in the third stanza:
No refuge could save the hireling and slave,
From the terror of flight and the gloom of the grave;
And the star-spangled banner in triumph doth wave
O'er the land of the free, and the home of the brave!
What is the effect of rhyming "save" "slave" and "grave" in this particular instance? How does that compare with the rhyming of "wave" and brave"? What is the effect of alliteration in "gloom of the grave"? What is being emphasized by using this alliteration? How does it affect the mood?
Does Keats use similar methods of creating mood in his poems? Or does he use other methods? Perhaps he uses assonance, or metaphor?
If you want to take it further, you could examine the meter of the poems. How many syllables are in each line? How many lines are in a stanza? Which syllables are emphasized? Where does the author change those patterns?
For example, Keat's stanzas are each 10 lines long, with all lines containing 10 syllables apart from the 8th line, which contains 6 syllables. Do the 8th lines bear a particular meaning in the stanza? Does Key pay as much attention to meter?
Hope that can get you started?