With Windows 8.1 (non-pro), there is no reason to choose OEM. The price difference is negligible.
With retail (FPP) version you can transfer it to another PC and you also have both 32 bit and 64 bit versions.
With OEM, the license is locked to one PC (and, at least here, the PC and Windows 8.1 must be sold together, on the same Invoice - otherwise it is the same as using a pirated version). And the OEM is either 32 bit or 64 bit (you can not switch later).
With Windows 7 (all versions) and with Windows 8.1 Pro, the price difference is bigger between same version in OEM and in Retail.