This is a no brainer for me. Windows 10.
There are too many reasons to mention.
http://www.marketwatch.com/story/microsoft-admits-failure-on-windows-8-2013-05-07
http://www.computerworld.com/article/2476464/microsoft-windows/did-microsoft-just-admit-windows-8-is-its-worst-operating-system-ever-.html
They forced users to use Metro, which took many more steps sometimes to launch the same thing as it used to in windows 7.
Most people purchased or used free or cracked versions of start button replacements for windows 8. Windows 10 has a real start button.
Windows 10 I have already installed on many computers and shows the same good performance and reliablity as windows 7 and 8 but with added usability and future proofing.
Dont wait...