Both are just fine for mining and the differences are marginal most of the time. After you get BIOS issues and all that out of the way, just pick whichever you like more. Windows is usually not free and although you can get licenses for under $5 nowadays, I still prefer Ubuntu for its simplicity and compatibility with pretty much all mining software.
Where are you getting a $5 Windows license?
Seuss
I get free.
Install 7; activate.
Install 10 as upgrade after SP1 updates on 7.
Automatically Licensed/activated copy of 10.
Just don't use a blacklisted 7 install; or janky autoKMS.
You don't even have to go through all of that if you have a valid Windows 7 key as the Windows 10 activation will accept those keys as well on a fresh install. I have done this on several of mine as well as several friends PCs already and it has worked every time.
I think installing Windows 7 for the sole purpose to upgrade to Windows 10 will have the potential to cause more issues with stability down the road versus a fresh Windows 10 install.
Also if you look around there are free options for Windows 10 as well such as the Assistive Technology Users Free Upgrade found here:
https://www.microsoft.com/en-us/accessibility/windows10upgrade