In response to your first question - I'd say expository is easier to score well in, especially if your SAC is under timed conditions. Audience and structure is pretty open, but make sure you have a good grasp of what audience and structure and form your piece will take before you go into the SAC (this will also save a lot of time if your SAC is under timed conditions).
Best preparation - I have my SAC on this context tomorrow. I've found that doing a shit load of mind maps for practice prompts and a heap of practice pieces very helpful. Also, talk to your peers about their opinions on the practice prompts as this will help you shape your own.
In the beginning, I also had trouble selecting what to write on. But just practice a heap and you'll be fine. Personally, I'd avoid a TEEL convention because this is not reading and responding. Avoid a TEEL structure like the plaque. Instead, find a style that best suits your writing style and your own voice as a writer, as this will allow your ideas to develop more complexity and be stronger than others who may try to follow a set structure.
Best piece of advice I got from my teacher for this context was this: (keep in mind I overthink - a lot)
"Don't overthink, just think and write."
I.e. trust yourself in the ideas you choose to explore from the text and trust your ability as a writer.
Good luck!