In comparing the themes of these two poems and finding similarities, I'd focus on how nature can be distracting. In "Stopping by Woods on a Snowy Evening," the speaker longs to remain a while in the serenity of the snow-filled woods, yet he knows that he has "promises to keep." Thus, he must continue on his journey, eventually turning from his distraction and back to his purpose. "The Road Not Taken" also presents nature as somewhat of a distraction. Consider the way the speaker hesitates from the earliest lines:
Two roads diverged in a yellow wood,
And sorry I could not travel both
And be one traveler, long I stood
And looked down one as far as I could
To where it bent in the undergrowth;
(Bold added for emphasis.)
This speaker also has a purpose: he is following a road en route to a destination when he becomes so conflicted by the way the paths diverge in a yellow wood that he can scarcely decide which road to take. In both poems, the narrators pause in their journeys because nature distracts them—in positive and negative ways.
Overall, the themes between the poems present quite differently. The speaker in "Stopping by Woods on a Snowy Evening" is filled with a clear sense of purpose. He hesitates briefly to enjoy a moment's peace, and then he recalls his mission and knows that he cannot fully rest yet. Compare that to "The Road Not Taken." This speaker invents difficulties in nature that never existed. The road that was originally described as "fair," "having...the better claim," and "grassy" (not rocky) is recalled "with a sigh" many years later as the road "less traveled by." The poem is even titled "The Road Not Taken," indicating that the speaker is forever filled with regret over the choice he didn't make or the road he never saw.
You could conclude that "Stopping by Woods on a Snowy Evening" is a sincere look at the beauties of nature, while "The Road Not Taken" uses nature to present a more sarcastic stab at human nature—we always want to believe that we have chosen the more difficult paths in life, even with evidence to the contrary.