On my system 8.1 is smoother than 7. It really depends on driver version. Every month I test the new drivers and the last one is the smoothest of all in FFXIV (nvidia GTX760) on Windows 8.1. So it comes down to the drivers mostly.
Windows 10 will be out next year and many features are left out from the tech preview so we are viewing an unfinished beta program yet. The final release will be much better.