The speaker in both sonnets is a man (presumably) who does not care about what a woman looks like, only how beautiful she is inside. He is mature enough to overlook physicality and focus on the sort of beauty that withstands the test of time. (He does, however, have an odd way of showing it in Sonnet 130, which in all fairness is not addressed to his lady, but to unnamed others who might presume to judge his lady based upon appearance and grace instead of character and intelligence.) He also believes in the power of verse/literature to immortalize her beauty, which suggests he is a man of learning, of letters.
In Sonnet 18, he points out that even a summer's day--a reference loaded with the happy connotations of comfortable laziness and beauty, as well as the height of a woman's beauty, if her life is a metaphorical changing of the seasons--cannot compare to his love's beauty. She is "more lovely and more temperate" (2), meaning that not only is she more beautiful, but she shows more moderation and self-restraint. A summer day can get too hot and summer ends too quickly (3-4), and weather is capricious ("every fair from fair sometime declines, / By chance, or nature's changing course, untrimm'd" (7-8)--"untrimm'd" meaning in this case "uncontrolled").
In contrast, his lady has an "eternal summer" (9)--she will forever be beautiful. The only way this is possible is if he is not referring to her perfect skin and teeth, rosy lips, etc. but instead of her inner beauty--her personality and character, her wit, her compassion. In a sense, he adds, she will not only be beautiful forever, but she'll never die because he has preserved her in his "eternal lines" (12).
In Sonnet 130, to make the same point (that inner beauty is what counts to him), he goes so far as to insult his lady's physical attributes. Her breasts are not white as snow--a common cliche by countless other "poets"--but "dun" (a greyish-brown color), if truth be told. She doesn't have hair like silk, but "black wires grow on her head" (4). Her breath is not sweet, but instead "reeks" (8). (Ew, right?) She isn't even graceful like a goddess, but "treads on the ground" (12), like you and I; he implies she may even be a bit clumsy. However, he does not care because, he says: "And yet, by heaven, I think my love as rare / As any she belied with false compare" (13-4). She cannot and should not be compared to women of mere physical beauty; such a comparison would be false, as her beauty and true value lies elsewhere.
Sonnet 130 also bespeaks the character of the poet/narrator in that he shows disdain for not only physical beauty in a woman, but in the silly cliches his fellow poets have employed to woo women for centuries. He's even a bit condescending to such "poets," in the same way any writer worth her salt is today when she reads comparisons so lazy as to have become cliches (like "worth her salt" ;) ).