This poem is not a sonnet. All forms of sonnets (English, Italian, Spenserian, etc.) have 14 lines. Also, the rhyme scheme is not that of a typical sonnet. The rhyme scheme is aaba, ccdc, eefe, gggg. The meter is iambic tetrameter (that is, four feet per line of unaccented/accented syllables). In the poem, the author is compelled to stop and contemplate the beautiful, dark deep woods on a snowy evening while he is headed home. He wishes to remain and drink in their beauty, but he has obligations - "promises to keep" and "miles to go" before he can sleep. This poem is celebrated for its seeming simplicity but depth of theme, rather like the woods themselves. The imagery is quite beautiful, don't you think? You almost feel as if you were there.
"Birches" is not a sonnet either (this is a bit more obvious). It is considered one of Frost's more popular poems. In this poem, the author is contemplating some birch trees that have been bent over by the weather, but he chooses to construct a more exciting story of how they got that way. So this poem explores the concept of reality versus imagination - what really happened as opposed to what the author would like to imagine has happened.
Can you find a similarity in the two poems, then? Both of them deal with opposing ideas - wanting to enjoy the peacefulness of the woods and having to get home to attend to business; admitting that the weather has made the birches bend over or imagining that a young boy has been playing on them.
You can get some additional ideas for your compare/constrast right here on eNotes.