Thank you bangla_lok for doing the legwork for me!

In case that information wasn't enough, to answer your questions directly:
- Both styles have their advantages and disadvantages. The expository style allows for explicit communication of ideas, but can be boring or pedestrian unless you're a good writer with something interesting to say about the Context & prompt. The imaginative style is risky given the amount of subtle 'show don't tell' requirements, as well as the difficulty of balancing the writing of a believable short story/ inner monologue/ w/e, with the actual Context task criteria. However, it gives you a much bigger opportunity to show off and write a piece that's enjoyable for you and your reader.
It basically comes down to what kind of good writer you are. If fluency and sophistication are your thing, then writing an expository piece will suit you well, whereas if you gravitate towards creativity and 'flare,' then an imaginative piece can better showcase your abilities.
What I'd recommend instead of just going cold turkey to a totally different style of writing, is to try a 'hybrid' piece. This is a very generic term to describe any 'in-between' sort of writing. It could be a predominately expository discussion about an issue in the media, presented in the form of a newspaper or journal piece; it could be a short story interwoven with expository reflections, or it could be a straight up interpolation between an essay's exposition, and a creative piece (as in, one paragraph on each with common threads between them.)
So there isn't really any restriction on what either style can do because you can call your piece a hybrid and get away with whatever experimental combination works best for you.
With regards to the marks, I remember being told that the averages for expository were pretty much an exact bell curve because it's the most popular style, and there's a lot of variation in ability across the state. Contrarily, the creatives tended to be (roughly) spread between 5s/6s and 9s/10s. Obviously they try to standardise these if there's enough of a margin, but generally speaking you'll either get a creative piece totally spot on, or off by a long shot.
If your teacher is pushing you down an imaginative path, then you should definitely give that a go for the SACs, but speaking from experience, trusting an expository or hybrid piece is probably best from an exam standpoint, but that's just my opinion
