From the previous suggestion:
There is zero reason to spend the extra for Win 7 Ultimate
There is little reason to spend the extra for Win 7 Pro, over Win 7 Home Premium.
Likewise, there is zero reason to buy Win 8/8.1 Pro over regular Win 8/8.1.
To your actual question:
There is currently little if any performance difference between 7 & 8.
8, obviously, is the direction MS is taking, so any future DX will be aimed at 8.