Notable: there are almost 3x more Windows NT devices online than Mac OS X 10.8
Also there are many valid reasons to not upgrade from NT...
Not if you expose your machine to the internet!
I suspect NT wins unless there are unpatched TCP/IP exploits.
I wonder what percentage of in-use ATMs make a similar bet.
There are some nt machines chugging along doing specific tasks that will outlive us all.