Direct X 12 is supported on Windows 10 while Windows 7 doesn't allow for it in spite of having hardware that can run it.
Regardless of what you do with your system, if you are on a system that is at least a Haswell/Haswell refresh system then it'd be a worth while endeavor to go for Windows 10. I would like to wait out for a little while longer to see if Microsoft decides to change their policy of constantly updating device drivers at i's own pace. The latter feature has been known to cause a stir up among the masses. For me, I'm sticking with Windows 7.