As others have said, sizing is more complicated than "I'm this tall, so I need this size". Some people prefer a larger frame, others prefer the smaller frame and longer stem/seat post, etc...

Conventional wisdom is that it is easier to make a smaller frame 'feel' bigger, than it is to make a larger frame work for a smaller person, so if you're torn between the 54 and 56, and can't ride them both enough to be 'sure' then choose the 54.

I am essentially your reported size (5'9, 31" inseam), but not everyone is comfortable the same way. In my experience, 58cm will be too big (I had a 58cm road bike that I could ride, but it never felt comfortable -- but you could be different). Even the choice between 56 and 54 (or near equivalents) is very personal. Geometry of the bike will matter a lot. Do you like being more 'stretched' or do you want to feel more relaxed? I had a 54cm Dew Plus, and when it walked away, I replaced it with a 56cm Dew Drop. I'm completely comfortable on my 56, but I can see where a slight change in geometry would make another 56 feel a little too big. OTOH, some 54s will feel too snug for me.