both. well. win8 is the only one having support for DX11.1. Win7 wil not have that support.
So, i'll say win8, but at this time there are no need to go rush for win8, it's still very new.
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I believe that Windows 7 handles game play and graphics rendering better than 8. Particular games on highest settings were noticeably laggy on 8, whereas 7 never gave me those problems.