Hey just a spoiler here. I've tested that and well a thinner rubber band does really have the same thrust compared to a thicker rubber band. Go with thicker.
Also if you need to compare the longer rubber band vs. smaller it can vary both ways. With a longer rubber band you can wind the rubber band even more except you have a lot of potential winds left. As for a shorter rubber band you will have a more powerful thrust but has less winds. Test for yourself. It depends what results your aiming for.
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