I'm 6' and prefer a 56cm frame. I think at 5'9" you'd be more comfortable at longer rides with the smaller frame. You're less stretched out so it's easier on your back.
You're in that middle ground like me but leaning towards the smaller frame is usually preferable.
This is the typical ratio for height to frame:
5`6`-5`9` 168-175 cm 54-55 cm
5`9`-6`0` 175-183 cm 56-57-58 cm
6`0`-6`3` 183-191 cm 58-59-60 cm
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