Windows 8. No question.
It's much better optimized and has the "swoosh" feeling. Once you get over the start menu now being full screen, its a pleasure to work with. Some things I now can never do without now that 8 can do that 7 can't, like native .iso mounting (if you game properly, you know why you want this) and proper copy-paste multithread management.
Also, all games I have work on 8. All of them. All 200 odd steam games I have work fine, including all the call of duties. If you ever do run into a problem, the win8 shell can emulate win7 call functions, so games will work with minimal tweaking (The only game I've ever had an "issue" with was Killing Floor LAN. All we needed to get it working was put it in compatibility for win7, and it worked right away).
That and the win8.1 start menu is animated! FRIEKING DRAGONS MOVE ALONG THE SCREEN AS YOU SCROLL! HOW COOL IS THAT!?