My friend decided that she would do short answer first and then multiple choice because if worse came to worse and she ran out of time she'd at least be able to guess the multiple choice, but she wouldn't be able to guess short answer. I recommend multiple choice first though unless you really think that you're going to run out of time for the reason Nick provided and because they can be almost treated as 'warm-up' questions to get you thinking and remembering information. Also, questions in the multiple choice sometimes relate to a short answer on the same topic and it helps to have thought about them before structuring your short answer response. Anyway, you should do the 2007 mid-year timed and under exam conditions and see how you go in a few weeks time or, even better, ask your teacher if they'll give you a practice exam in class (and will correct it for you!).