Fence

If you are on the cusp between two recommended sizes, it's best to try riding both. At a minimum, check the stack/reach measurements of your current bike and compare them to the new model you are looking at to get a feel for a fit you are already comfortable with. If riding the bike is not an option, consider the following.

Personal Preference
A larger frame will be more stable and give you more room to move around without upsetting the weight balance of the bike. The larger size will place the front wheel further out in front of you giving you a feeling of security, and conversely will require more significant body movements when you DO want to change the weight bias of the bike. Think carefully about your riding style and how active/responsive you want it to be versus stable and fast. You will have to work a bit harder to move the bigger bike around.

Body Dimension
Not all bodies of the same height are alike. If you have longer legs and a shorter torso than average for people of your height, you may prefer the smaller of the recommended sizes. If you are all torso and arms, then you will likely want to size up.