What are some similarities and differences between Shakespeare's Sonnet 18 and the balcony scene in Romeo and Juliet?

For starters, both Sonnet 18 and Romeo and Juliet were written by William Shakespeare! Though this fact makes further similarities and differences all the more interesting. Let's consider some differences, first.

While Sonnet 18 is a piece of standalone poetry, the balcony scene of Romeo and Juliet is but one part of a greater play. What's more, Sonnet 18 is written from the perspective of the speaker to an unknown reader. In this way, it is like a love letter. We have the perspective of the speaker, but not the recipient-subject. The balcony scene of Romeo and Juliet may be considered to consist of parallel monologues until the two characters actually speak to one another. It differs from the Sonnet because we hear both characters' perspectives, and each is the recipient-subject of the other's speech.

What do these two pieces of text have in common? Both are declarations of love! They also bear similarities in their content. Comparisons of beauty and nature are in both, evoking images of beautiful summer flowers. One can easily understand what Shakespeare found beautiful in nature as well as in a person by reading and comparing these two texts.