I recommend OEM. Perfect fit. I replaced my front and rear non-M bumpers with OEM M bumpers.
http://f10.5post.com/forums/showthre...ghlight=bumper
With straight after-market, you may save money up front but spend more on installation labor if you come across fitment issues. With OEM, you WILL spend more up front, but you will have a very low labor cost for install because fitment will be perfect.
There is a third option. Insurance companies authorize collision repairs to be made with Assured Quality Replacement Parts (AQRP). These parts are certified for a higher degree of quality and fitment than we would normally be able to get from eBay. Here's the link to a National business that wholesales and retails AQRPs.
http://www.lkqcorp.com/
A local collision repair center may also give you a good price on labor if you let them make a little money on buying the AQRP bumper for you.
Enjoy!