These are two very good poems to compare and constrast. If I were you, I would approach this question by thinking about what these poems say about death and how death is presented in both. This is the thematic link, if you like, that you can use to compare and contrast them.
"Because I could not stop for Death" features the character of Death personified as a carriage driver who kindly stops to pick up the speaker. As the journey progresses, the passenger feels steadily colder, until finally the carriage pauses at her graveside and the passenger realises that the carriage ride has been going "toward Eternity." There is intense irony in Dickinson's presentation of death and the process of dying, focusing on the gradual comprehension of the speaker and what is happening to her and the presentation of Death, normally so fearsome and scary, as a polite carriage driver. Note how the last stanza couples two differing tones regarding death and the speaker's acceptance of it:
Since then--'tis Centuries--and yet
Feels shorter than the Day
I first surmised the Horses' Heads
Were toward Eternity--
On the one hand, there is the tone of terror in that one day of the speaker's understanding seeems to last longer than centuries. However, there is also a tone of calm acceptance, as we are told that the centuries have passed quickly. Thus Dickinson in her poem presents death in a way that demythologises the frightening figure we normally associate with death and explores how sometimes death and our understanding of it can creep up upon us unawares.
"Stopping by Woods on a Snowy Evening," in contrast, is all about the desire for the rest and lack of responsibilities that death brings with it. It is important to realise the symbolic significance of the woods in this poem, as featured in the last stanza:
The woods are lovely, dark and deep,
But I have promises to keep,
And miles to go before I sleep,
And miles to go before I sleep.
The speaker desires to stay in the woods, that are described as tempting and almost seductive. However, he still has responsibilities at this stage of life, featured in the "promises" he has to "keep," and note how the repetition of the last line reinforces the way that his life is not over yet whilst at the same time presenting death as a "sleep," as an attractive stage of life that is to be yearned for as a time when there are no more "promises to keep" and the body can find rest in a beautiful place.
Thus what unites these poems is their novel presentation of death, not as something to be feared, but as something that is to be yearned for on the one hand, and something that we are surprised by on the other.