I hear a lot about test riding bikes and picking what feels best for you. Problem is, unless you are a an experienced bicyclist, this oft recommended tip won't work. Since when does a bike shop take the time to custom fit a bike to fit you? They may adjust the seat height but that's about it. What fits you is totally arbitrary to the particular setup these bikes happen to be in. My tip is, unless you know your body varies from average (ie. short/long legs, short/long upper body, etc) or are particular about either being fully upright or stretched out on a bike, follow the manufacturer guidelines. If you are close to a larger size, pick the larger size.