For Keats, a good bet is the article by Helen Regueiro Elam, "Remembering to Die." Here is the abstract. A link to the full article is listed below.
Critical tradition has tended to read this Ode as the poet's triumph over the power of mortality—a power by which he is besieged and to which he responds in his other Odes. This essay suggests that especially in this Ode, where the art object places itself "far above" passion and death, Keats is intensely aware of the indispensible power of mortality as the very source of eros and art.
For Sonnet 116, consider looking at one of my favorite critics' work, Helen Vendler. She has an excellent book titled, "The Art of Shakespeare's Sonnets." An excerpt is below, but you may want to see if your high school has a copy. You may also want to see if your library subscribes to the literary journal JSTOR for a more exact criticism of this particular sonnet.
I have also listed the link to analysis of the sonnets here at eNotes for you.
The criticism is immortality .Shakespeare in sonnet 116 makes love immortal .He tries to establish the idea that love is not Time's fool.
Keats in Ode on a Grecian Urn immortalizes art .Art is unravished bride of silence and slow time .Time fosrers art , and grows with Time , remains with time .
Shakespeare makes Time carry love .While ,Time destroys all other things of life , in that case , it can never find love within its sickle .Rather Time considers it blessed , as it can bear the triumphant flag of love through its passages .