This is completely up to you, based on personal preference. Most of our customers prefer the upper guard as they find that it's less in the way of the tongue while sleeping or talking. Some of our customers opt for a lower guard if they've had dental work done (crowns, implants, etc), or if their dentist specifically recommends so.The good news is that you only need one guard to protect against grinding, as one is enough to act as a protective barrier between both teeth. However, you may order both if your dentist recommended you do so.