One thing to consider - Windows 10 is free all year long to everybody, even those who pirate windows 7 or 8. There really is no reason to buy the windows version at least for a year, because you can easily install windows onto these if you want for free for the next 12-ish months.
Keep in mind that you can also dual boot your steam machine. Mine has two OSes installed - Windows 7 and SteamOS.
The way you worded it - is w10 subscription based?