Based on the download counts on the
Armory downloads page on Github, the last release of Armory was about
70% Windows
30% Linux
0% Mac [not supported!]
However, the stats are a little fuzzy because many users will actually install Armory on two different computers (online and offline), and many users download two different versions, maybe 3 or 4 if they get the versions wrong. I've been recommending using Ubuntu for the offline system, because it's very simple to setup a linux system that only runs one program, even if you're normally a windows user. That may be skewing the results a bit in favor of Linux (actually, does it matter? They are setting up Bitcoin software on a linux system, maybe they should count anyway...)