The comparison essay in English is often seen as the most difficult and scary essy. However, it can actually be an ideal opportunity to get your own thoughts and inspiration across. I wrote about Milton's Paradise Lost and Webster's The White Devil. The key thing is to select a few key themes and go into them in detail.
Comparing specific words is a really good way or getting that detail required - think about sentence structures, alliteration, similes and metaphors and if there is a deeper meaning that you can connect. Also incorporate critical analysis into the answer - and then make sure to analyse that. Examiners would you prefer to talk about less things in more detail than to shallowly cover many topics. Talking about the different types of writing is an obvious, but perhaps overlooked, detail - writers place importance on stage directions (if a play), line length (in a poem) and syntax. Providing insight into these types of things will help you gain the best mark.