I think a good reflector/light medium should be chosen on more variables than reflectivity alone. I am not an advanced indoor marijuana grower but I work for an Air Conditioning contractor and do all the high voltage/Controls and interface devices. An my grandmother has owned an organic nursery since I was a small child. I fancy myself as being mechanically and horticulture inclined.
As far as material is concerned IMHO something white can only be a good thing. Being a man of the trade, I have spent many a year on a hot industrial roof in Florida. Sheet metal/Flashing or Mylar type roofs are always very bright and ungodly hot. The newer style poly-white or white mastic painted style roofs are super bright and are cool enough to lay on in the pitch of summer. They are notorious for burning under your chin and the bottoms of your cheeks from the excessive amount of radial reflection. The mylar-like/shiny or silver rock roofs reflect a lot of light but hold every bit of heat. With proper cooling/ventilation either choice is good though.
Like I said, I am no master indoor gardener but from my experience on a much larger scale, White is right!
