Custom dielectric mirrors can be manufactured when you are aiming for high reflection at specific wavelength or angles of incidence. The advantages of dielectrics are they are tough and can offer higher reflectivity then metals at specific wavelengths and angle of incidences. It is also possible to cut down the reflectivity of other wavelengths simultaneously acting as a filter. The disadvantages of dielectrics are the fact they are so wavelength and angle of incidence dependent. This means if you are needing performance over a large range of wavelengths and angles you are most likely better served with a metal mirror.
Protected aluminium is the most common mirror coating material. It offers a reflection of approximately 86% through the visible while being a tough and resilient coating. The majority of optical mirror applications will be well served by using protected aluminium.
Enhanced aluminium is the process of putting a multilayer dielectric on top of aluminium. This can increase reflection performance up to the 94-95% while still being tough and resilient.