I used the SI mirrors. My research showed:
Si Mirror: Silicon Glass Gold coated, great reflective index, not good for anything 80 watts or over (not too hot on 60 watts either)
Mo Mirror: Molybdenum, very tough but the lowest reflective index, great if you miss cleaning or have a habit of dropping mirrors or if you have over 80 watt sources.
Cu Mirror: Copper, Below gold Si for reflective index but tough and better index than Mo
As for coolant, I use distilled water and add some propylene glycol (not ethylene glycol - I have animals and they are less likely to drink in case of a spill). DI water can be very aggressive towards some materials. For example, I have a cooler on my laser that has copper pipes that carry the water. DI water will corrode copper. to quote: "Care must be exercised when using DI water. The very lack of ions also makes this coolant unusually corrosive. Called the "universal solvent," DI water is one of the most aggressive solvents known. In fact, to a varying degree, it will dissolve everything to which it is exposed. Therefore, all materials in the cooling loop must be corrosion-resistant." - source:
http://www.lytron.com/Tools-and-Technic ... ized-WaterScott