To answer this question you need to be aware of what these two characters symbolically represent in the novel as a whole, and how they reflect a conflict that is played out within Jane herself. One of the key points of dialectical tension in the novel is the struggle or conflict between passion and sense, between emotions and duty. Various characters reflect either one of these extremes to varying extents. Thus, for example, Helen Burns is an extreme of sense and duty, a model of restrained emotion, whereas Mrs. Rochester (the first) is an example of passion run amok.
By this way of looking at the novel we can see that Rochester can represent passion and some of its excesses, whilst St. John represents sense and duty. For example, Rochester as a Byronic hero is a character who has quite a colourful past - he has had a string of mistresses and is obviously sexually experienced. Thus when Bertha Mason burns his bed it is symbolic of passionate excess on his part.
On the contrary, St. John Rivers is excessively fixated on duty. So much so that he forsakes the woman he loves to pursue his dream of going to India and working there as a missionary. It is interesting to focus on how St. John is described. The word "marble" is used frequently, as are other descriptions focussing on cold imagery:
...the cloak that covered his tall figure all white as a glacier.
Such descriptions reinforce the coldness of his personality - he is duty taken to excess, just as Rochester is passion personified.
Of course, what you will want to think about in your essay is how these two opposing forces play out and demonstrate the conflict that goes on within Jane herself as she wavers between these two emotions. She recognises the danger in both, which is why she rejects becoming Rochester's mistress and rejects marrying St. John for duty alone. It is highly telling that she only married Rochester once he has gone through a purgatorial experience and has been maimed. Thornfield could be said to be a symbol of Rochester himself, so with its destruction we see a much more tamed Rochester at the end of the novel (he is compared to a caged eagle with its wings clipped), and thus is ready to marry Jane, who in so doing manages to finally reconcile the fight between passion and duty in herself.
Hope this helps - a few ideas for you to chew over!