There is also a big difference in support. Support for OEM products is provided by the seller (vendor), not by the manufacturer, i.e., if it fails you have to send it back to where you bought it from. Support for retail products is provided by the manufacturer.
Sometimes it is worth the additional cost to buy retail items, especially with something like a high end motherboard. You just need to look at each purchase on a case basis to determine if it is worth paying the additional cost for the retail version.