OEM version or Full version?

Solution
Well, depends. If you plan to just stick to that build for a while, OEM is a great cost savings. If you plan to continuously upgrade such things as your motherboard, between Windows release cycles, go with retail.
Actually, if you want to get technical, you're not even supposed to use anything but retail versions unless you're a system builder.

Windows 8 and Windows 8 Pro are the first Windows product to allow for a personal use license with OEM software. But it only qualifies on new builds, not upgrades. Windows 8.1 does not qualify for the personal use license, but when upgrading from 8 to 8.1 the personal use license rights transfer.

If you are purchasing Windows 8.1, you need full retail. (pardon my brain fart) There is no retail for Windows 8.1, as it's intended for OEMs. If you're purchasing 8, you can choose OEM for a new build, then upgrade to 8.1 for free and your rights will automatically transfer.