For a laser with less than 80W and clean engraving jobs I would recommend the Silicon-Mirrors because they have the best
reflectivity (>99%).
For a laser who is doing very dirty work (cutting wood 8 hours a day) or working with aggressive chemicals I would recommend the
Molybdenum-Mirror. The reflection coating (there is no coating, the Molybdenum is just polished) is very resistant and can get cleaned
very easily. Even if the reflectivity is only at 98.5%, the Mo-Mirror is almost unbreakable.