Windows 8/ 8.1 is recognized to be a better base OS for gaming at this point.
In reality the difference is never more than 1-2 FPS higher or lower than 7, so as far as a perceivable difference goes, there isn't any.
That being said, Win8.1 boots and shuts down quicker than 7:
Windows 8 boots faster because Microsoft wrote it so the OS and motherboard never really shut off. Windows 7 writes itself to the drive then shuts off. With windows 8 it writes itself like it sleeping on power down so it never realy reloads itself at cold boot.