Snow is not the best reflector, file it under useless info.
If you paint aluminum white, it will reduce the light reflected, total and spectra.
Mirror is the best, it reflects almost all the light spectrum accurately.
A properly shaped polished reflector will reflect more total light than a flat mirror.
For VHO think about mirror acrylic. Look for it at any plastic/glass shop, it's cheap too.
For MH think about a 12 inch mirror tile from home depot, it will take the heat just fine.
You stated you have a VHO setup. Think about getting the bulbs with the 180 deg internal reflectors, thereby making a reflector unnecessary, although may not be avail in your length.