Both poems are sonnets, and both are about love. Shakespeare’s speaker describes his lover in metaphors and she seems to return the affection, but Spenser’s speaker describes a lover that does not.
In Shakespeare’s “Sonnet 18,” the speaker compares his love to a summer’s day and says she is more beautiful. Since he says she is “more temperate” we can infer that she has a good personality and a sunny disposition. She is, in essence, sunnier than a sunny day.
When in eternal lines to time thou growest:
So long as men can breathe or eyes can see,
So long lives this and this gives life to thee.
Unlike a summer’s day her love will never end. They seem to have a good relationship, and the speaker is convinced that their relationship will continue.
Poor Spenser’s speaker is not as lucky. Unfortunately, no matter how he tries to woo his love she spurns him. He cries, and she says his tears are just water. No matter what, she is as hard as flint and he can make no progress.
The firmest flint doth in continuance weare:
Yet cannot I, with many a drooping teare
And long intreaty, soften her hard hart
Unfortunately, the speaker’s love does not return his affections and no matter what he does, he cannot soften her hard heart.
Besides the two types of sonnets, these sonnets are very different in content. Both describe speakers who are hopelessly in love with their lovers, but Shakespeare’s speaker’s lover is a sweetheart, and Spenser’s is a tease.