Seeing as you are a Yr 11 student I think you should go for posters if you are having trouble understanding/remembering content in physics.
Although Physics is a math/science subject and is way different to subjects like History etc. I still think it would be good to create posters so you can include diagrams. This will allow you to gain a conceptual understanding of the content which I believe is very important in Physics. For tests and exams, I would lean towards cheatsheets, since that is what you will be allowed to take into your final Unit 3/4 examination.
Upon further reflection, since you have time, experiment with both and see what works for you. If you feel you should only go with one of the two, go with cheatsheets. It is crucial that you emulate exam conditions so you are prepared and confident for your final exam.
Hope this helps
