OEM comes with no fan and a 3month warrenty or something like that. If your cpu works for 3months it will most likely work for 5 years. And if you buy OEM, you get to chose your own heat sink fan like that if you want to overclock and stuff. Retial comes with a sticker, so if a 3 year warrenty, the AMD sticker, and you don't plan on overclocking, then get retail. If not get OEM.
Either way you can't go wrong.