William Blake's "London" and Wordsworth's "Composed upon Westminster Bridge, September 3, 1802" are both about London. Blake's poem is four stanzas of four lines each and Wordsworth's is in sonnet form.
In Wordsworth's poem, he favorably describes London as it appears early in the morning. He begins quite dramatically, saying that there is no sight "so fair" than this image of London in the morning:
The City now doth, like a garment, wear
The beauty of the morning; silent, bare,
Ships, towers, domes, theatres, and temples lie
Open unto fields, and to the sky;
The city wears the morning's beauty like a garment in the morning, before people are awake, before the noise and erratic movement of city life. It is also before the factories and furnaces are churning out smoke into the sky. The speaker notes that the sun shines more beautifully on the buildings than it ever has on valleys, rocks, or hills. This is ironic because much of Wordsworth's poetry is about the beauty of nature and its lack of industrialized distractions. However, it fits with Wordsworth's love of nature that the city is quite literally asleep and is therefore as calm as a scene in nature. The 12th line does directly address nature, noting (at least at this hour of the morning) that the river moves freely.
Blake's "London" describes the city quite negatively. He notes that each street is "charter'd" which has a double meaning: "given liberty" as in a charter, but also noted as private property and therefore rented out. This is significant to the rest of the poem because Blake comments on how the less fortunate are at the mercy of institutions, government and church.
In the second line, he also describes the Thames as "charter'd" meaning that it flows freely (given liberty) but also that it is the property of the state. This latter meaning contrasts with Wordsworth's notion that the river "glideth at its own sweet will:" - whereas in Blake's poem, the river is owned.
Whereas Wordsworth sees beauty in the city's morning, Blake sees "weakness" and "woe" in every face. Blake notes the limits and laws that act like "mind-forg'd manacles" and these include bans which can be legal or political prohibitions (ban can also mean a marriage proclamation, so Blake indicates that this arrangement is also like a business deal and/or a sentence).
In the last stanza, Blake (or the speaker) notes that at midnight, he hears the prostitutes curse their clients. The curse "Blasts the new-born infants tear," and this indicates that the prostitute, having contracted a venereal disease will pass it on to a man who will then pass it on to his wife; thereafter, the child (of the man's wife or the prostitute) will have a blind child as a result. With the last line, Blake gets another jab at marriage which, as a ritual of an institution (the Church of England) much like a law, people becoming indebted to a certain way of life.
Clearly, the two poems paint very different pictures of London. Blake's is bleak and Wordsworth's is full of beauty. However, Wordsworth is describing London in the morning, prior to the daily life of the city. Blake notes in the final stanza that he is viewing the city at midnight. Had Blake been writing at sunrise and/or had Wordsworth been writing in the evening or at night, their interpretations might have been different.