Format is equally important. Your essay needs to have an introduction, reasoned argument and a conclusion.
Titles don't matter, about half of those on my AIB had pre prepared to answer a question on strategic nuclear deterrent, the gasps of dismay when this did not feature in any of the topics was amusing to say the least.
In terms of subject matter your should aim to be knowledgeable and conversant on current affairs and particularly defence matters in current affairs, to do this you need to be reading a quality newspaper (news website) on a regular basis. This might also help in the interview where these subjects may be (but are not guaranteed to be) brought up.