Looking at the chart, the 57 has a 1cm longer top tube, so the difference is more in the height. I'd size it based on how you want to set it up -- what bars are you planning on using, and how much seat to bar drop do you want? I'd go 55 if you want to do deep drops or bullhorns, and 57 for a more standard setup or a riser bike. 57 for sure if you have long arms (ape index over 1.5 or so). It's not a huge difference either way, 1cm at the TT is not much and at your measurements the standover doesn't matter much between the two sizes.

I wouldn't even buy a stem until you choose your bars, and then I would find a shop that has a good stem return policy.