Unless you have been specifically advised by your dentist as to the requirements of an upper/lower & thickness it really comes down to personal preference. If you are still unsure while reviewing our products we would always recommend speaking with a dentist before making a purchase. Predominantly customers find the Upper more comfortable and the impression process can be a little easier.If you have not used a night guard before we could suggest starting with a 1.5 or the standard 3mm again unless otherwise advised by your dentist. The 1.5mm is only suitable for mild grinding, the medium is mild to moderate. Once you have become accustomed to the wearing of a night guard and you feel you need something more substantial in terms of thickness/durability this is then where the HARD or PLUS can become beneficial.Nightguards do not come with a warranty cover. The longevity of a nightguard is very unique to each individual, although we provide a range of thicknesses to help support all requirements it is impossible for us to say how long this will last per customer as all grinding/bruxing is different in person to person. If there is a fit issue within 30 days we will of course assist in getting these rectified for you but will require the return of the nightguard to do so.