If you're between two sizes, more than likely, you should look at a different brand/model...
Also, recommendations such as "if you're XX" tall, use size XX frame" is not worth much at all... Size medium from one may be very similar to a 53cm from another, 55 from a third, 51 from a 4th, small from a 5th etc. There's simply no standard as to what a size denomination means. Even if you are looking at a chart from the manufacturer, keep in mind that for example a 530mm top tube is not always the same as another manufacturers 530mm top tube... how the measurement is taken can change the fit significantly, ex. center-to-center, center-to-top, center-to-virtual point, virtual point-to-virtual point... again, no standard how something is measured.
And that's before you add the rider... height is a pretty poor indicator of fit. Add upper body measurement, arm length, upper leg, lower leg, core strength, flexibility, athletic background, cycling experience, types of racing/terrain (you're not going to set up the bike the same if you're only doing short races, such as 1hr crits, versus multi-day stage racing or randonneuring, nor would it be the same if you riding in pancake flat terrain where wind will be your #1 issue versus riding in the steep mountains). Then add prior/current injuries/issues to that entire mix.
Basically, you take take 3 people of exactly the same height, one fits size medium of a specific model/brand, the next person fits the same model/brand but needs a size large, and the 3rd doesn't fit that model/brand at all, but fits another brand/model in size 53cm.
"Suddenly the thought struck me; my floor is someone elses ceiling" - Nils Ferlin