the main difference between OEM and Retail is that OEM has no packaging or instructions (other than a quick start), there is no product support and you cannot transfer the operating system to another pc eg. you buy a new computer without windows you cannot install a previously installed OEM version on it (well your not supposed to)
I think it has to do with computer builders for easy, quick, efficient installation. So I would always buy the Original.
Retail version has product support, packaging and you can transfer it to