Like zoog said, you can only use DirectX 11.2 with Win8. That's probably going to become more important in the future. For gaming longevity, you should go with Win8.1.
adamtron :
Im not much of a Windows user (Linux mostly), but I personally prefer windows 7. As do most gamers. It is (in my opinion) easier to use, less bloated and less invasive. I recommend trying both though, if that's possible, it is almost entirely preference.
Not true. Windows 8 is actually faster and more efficient than Win7. You can install Classic Shell for free, which bypasses Metro, goes straight to Desktop Mode, and gives you a Start Button. Going this way gives you the familiarity of Win7 + the nice enhancements of Windows 8. I always tell people that Windows 8 isn't worth paying for if you have Windows 7, but if you're doing a new install, definitely go with Windows 8 + Classic Shell if you don't like ModernUI (Metro).
zoog18 :
If I choose to purchase windows 8.1 and install from scratch, do I need an older version of windows installed to install it, or can I just install it fresh, and does the same apply to windows 7?
That all depends on whether you buy the full version or the upgrade. A long time ago, Windows upgrades used to let you enter a previous valid Windows license during the upgrade process so you didn't have to have it already installed. They don't allow that anymore. If you buy an upgrade, you have to have a previous version of Windows installed.