It is all about research. However, I haven’t found an extensive comparison so far. Apart from the link mentioned by @ganget, this link helped me dig deeper in the glass topic.
I’m trying to find a glass for my MagicMirror as well. I ordered the sample set from brigla and two 3mm samples with 12% and 20% light transmission from glaströsch.
To be frank I don’t find any of the samples satisfying so far. Compared to my bathroom mirror they all seem pretty dark. See for yourself. My favorite so far is the Mirastar 4mm.
In real life it even looks a bit darker than on the pics…
Mirropane Chrome Spy 4mm
Light transmission 8%
Light reflexion ?
0_1553347033532_59d76f32-71f6-4548-9aac-d5b21beb8e89-image.png
Pilkington MirroView 6mm
Light transmission ?
Light reflexion ?
0_1553347284729_9f41839e-4537-41e0-bb3d-f2eba7a08377-image.png
Mirastar 4mm
Light transmission 3%
Light reflexion >58%
0_1553347485964_77d5b607-c6ee-493a-8c30-fcdafb53919a-image.png
glaströsch Spionspiegel 3mm 20%
Light transmission 12%
Light reflexion 35%
0_1553347665325_cce58dbb-6fc6-4518-8214-58c1b10a1300-image.png
glaströsch Spionspiegel 3mm 12%
Light transmission 12%
Light reflexion 46%
0_1553347732711_5d852219-1f86-4cc8-b833-bea43b673927-image.png