OEM glass is Pilkington and is very soft and chips easily. (see Euro regulations re: glass safety). A/M glass has gotten much better; if you want A/M stick with PPG/ (pittsburgh glass works) which also does alot of OEM work. Definitely inspect the glass prior to install as you'll be able to see if there's any imperfections/distortions in the glass. If the A/M glass is good, just use it. The only downside is you lose the BMW logo in the lower corner of the glass.
So, while OEM is def #1; PPG or aftermarket Pilkington, is just as good IMO.
Make sure the glass is correct with rain sensor/etc prior to install also.