I'm sorta a fence straddler when it comes to the whole memorising/not memorising debate. I have always been told not to memorise essays, so I never did, and I did reasonably well internally in my HSC, I came top 15 at a top 10 school, and was extremely happy with my mark. In the stuvac period, I decided to take a risk, and since most of my friends were doing it, I thought I would be a sheep and copy them, so I memorised my essays as well. I did exceptionally well in the HSC, I managed to get 98 external.
So, what does this mean? To be honest, I have no idea. Was I just lucky and managed to memorise essays which were exactly those in the question? Or was a I good at adapting my essays on the spot? I have no idea. And I will never know. So my best advice - do what you feel comfortable with. If you're typically a touch and go, adapt on the spot sort person, do that. If you're a memorising person, do that. Whatever you do - know that you have to answer the question at hand, either by writing your essay on the spot, or adapting your memorised essay to suit the question.