Two things that both poems have in common, of course, are that they are both written by African American writers and that they both address the place of black people in America in one way or another. I would focus on these elements first in my comparison of the two poems.
Phillis Wheatley's poem "On Being Brought from Africa to America" seems in many ways different from Langston Hughes' poem "I, Too." Where Wheatley's poem presents blackness as a mark of sinfulness and as reminder of a past that is best left behind, Hughes' poem touches at least briefly on the idea of blackness as beautiful. Whereas Wheatley's poem seems to suggest that blacks should be treated fairly for religous reasons ("Remember, Christians, ...") even if they remain slaves, the note of protest in Hughes' poem is much louder (he wants a place at the table) and much more secular in nature.
Both poems have a serious tone, at least at times, but that tone is much more dominant in Wheatley's poem than in Hughes' poem.
I hope that these ideas get you started!