You want a bike that fits. Bikes have multiple adjustments so that they can fit different people. Your LSB is right that you are probably in the middle zone between a large and a medium. Adjusting seat height is easy, so that won't be an issue. If your inseam is 30 inches, then you will be able to stand over the bike comfortably. If it's longer, you can raise the saddle. The issue will be the length of the top tube. If you have average or short arms (32- or 33-inch sleeve length), then the medium is probably your best bet. If most of your height is in your trunk and have longer than average arms for your height (34-inch sleeve), then maybe a large is better. Here's a test: Place your elbow at the tip of the saddle and reach toward the bar. If your palm reaches the stem, then you're good. If it goes beyond, the bike is probably too small; if it doesn't reach, then probably too short. This is an inexact method but gives you a general sense. Try it on several bikes.