There are two different ways one could approach writing a compare and contrast essay about Sense and Sensibility by Jane Austen. The first is comparing elements within the novel with each other and the second is comparing Sense and Sensibility with another novel.
The most obvious internal comparison within Sense and Sensibility is of the two sisters, Marianne (characterized by her intensely emotional "sensibility") and Elinor (characterized by self-discipline and rational "sense"). Another possible pair of characters to compare would be the romantic but eventually selfish and unreliable Willoughby and the steadfast and patient Colonel Brandon. It would also be possible to compare the impoverished but still ethical and genteel Dashwood family with the vulgar and self-serving Steele family or Lucy Steele, fiance of Edward, with Elinor, who Edward eventually marries.
Another possibility would be to compare Sense and Sensibility with Pride and Prejudice which has a similar theme of various pairs of lovers whose eventual fates are tied to their moral characters and degrees of self discipline. The perfidious Mr. Wickham is very similar in type to Willoughby and Lydia Bennett could be compared to Lucy Steele. It might also be possible to compare and contrast how Austen portrays wealth and its influence on morality in the two novels.