Both of thees two beautiful sonnets deal with unrequired love; I would suggest using that as the basis for your comparison. Both speakers also deal with their sorrow over unrequired love by taking solace in being alone. Look for specific lines that emphasize each speaker's pain; for example, in Sonnet 28, the speaker is "anxious to shun the print of human tread (wants to be alone)...From the world's prying eye to hide my woe" (wants to hide his sorrow from others). In Sonnet 131 the speaker also describes his "discontentment in this one short life." However, in this poem he wants to achieve fame by writing of his love so that his words will outlive him--a frequent concept developed in Renaissance sonnets.
Keep in mind that when you write a comparative essay, you are also contrasting. You find a point in common, in this case, the unrequited love and each speaker's resulting sorrow, and then you point out how each one develops this idea differently (Sonnet 131 looks to achieve "glory in [his] later fame."). The speaker in Sonnet 28 prefers to take solace in nature ("That well I think each mountain, wood and plain / And river knows, what I from man conceal"). Once you have the basis of your comparison, you can look even more closely at the rest of each sonnet's lines to see how each speaker handles his unhappiness.
Good luck with your essay!