[slice of heaven]

Confusing? no , we haven't started yet.
The main difference in the type of glass used in the mirrors is how quickly the mirror cools and how much distortion of the mirror surface is created while it is cooling. Theres more than 3 types but we'll stick with these.
Plate glass,BK7 and pyrex.
Pyrex cools quicker and you have less distortion.
Plate glass cools slower and distorts the most while doing so.
BK7 is in between.
Once there cooled and if your not standing in a snowfield it doesn't matter.
A mirror made of BK7 with a fan to reduce cooling times means your mirror will reach equilibrium with the outside temp quicker and your viewing session starts sooner.
Or put your scope with aplate glass mirror outside earlier as it takes longer to cool.
The reflective coating is on the front of the mirror, not behind the glass ,as per a normal mirror. Light doesn't travel through the glass so there is no difference in reflective quality.
As for cooling fans, you can fit a cooling fan from a computer to cool your mirror if your handy enough.

Slice