In Sonnet 18, the speaker describes how the person he addresses is more sweet, temperate and fair than the beauty he sees in nature. He even notes how the sun is sometimes dim and how nature’s beauty is sporadic.

And often is his gold complexion dimm’d;

And every fair from fair sometime declines,

The speaker concludes that the beauty of the person he’s addressing is not so fleeting because it will live as long as there are people to read this sonnet. His beloved’s beauty last longer than nature because it is immortalized in verse. This lifts her to a goddess-like status.

So long as men can breathe or eyes can see,

So long lives this and gives life to thee.

In Sonnet 130, the speaker takes an oppositional or ironic approach. He notes how his beloved does not compare to the beauty he sees in nature. Music is more pleasing than her voice. Coral is more red than her lips. The speaker is chastising other poets who describe the one they love with exaggerations that are so over the top, they are “false comparisons.”

My mistress, when she walks, treads on the ground:

    And yet, by heaven, I think, my love as rare

    As any she belied with false compare.

The two sonnets are similar in that they compare a loved one’s beauty to the beauty of nature. Sonnet 18 is effusive and traditional. Her beauty is more impressive than nature and is immortal through this verse. Sonnet 130 is ironic, satiric and literally more down to earth. While many poets have described loved ones with goddess-like qualities, the speaker in sonnet 130 is much more honest and practical. In fact, you could say that the speaker in sonnet 130 is challenging speakers in other poems, like the one in sonnet 18. It’s like he’s saying his loved one is just as rare and beautiful: he doesn’t need to make exaggerated comparisons to prove it. Shakespeare shows his versatility and/or the willingness to mock others and himself.

Both of these sonnets address the speakers love for a woman.  In sonnet 18, the comparisons are almost hyperbolic.  The woman is compared to a summer's day and the beautiful summer's day is found wanting in comparison to the woman.  This poem is written to immortalize the woman.  "As long as men can breathe and eyes can see, so long lives this (the poem) and this gives life to thee."  The comparisons, in some small way, can be seen as a comfort to a woman who may be facing thoughts of her mortality.  The speaker reassures the woman that she will not be conquered either in death or in the dimunition of her beauty because the poem immortalizes this moment of her perfect youthful beauty.  In the second poem, Sonnet 130, we see a more realistic speaker who, instead of trying to immortalize the beauty of his beloved, insists that her beauty is not why he loves her in the first place.  Instead of flattering her vanity with false or insincere comparisons to nature's wonders, he insists that it is not her appearance at all that he loves but rather her intrinsic nature or personality.  She is not more beautiful than other women physically, but she is perfect for him and he refuses to falsely flatter her.  "And yet, by heaven, I think my love as rare / As any she belied with false compare."