Windows 8 is pretty good at handling system resources and being more efficient. However on a performance test above Windows 7 also does an excellent job. When Windows 8 becomes more widely common, system and graphics card drivers will improve giving it more support and performance in games.
In my opinion though, I would go with Windows 7. It is still fairly new, still performs, has plenty years of support left for new hardware and has a bigger user base making information and knowledge much easier to find if you have any problems.