Welcome!
Here is the break down (bad word I guess)
2008-2009 - Early X6s. Old iDrive, terrible rear view camera (and the camera is IMPORTANT on this vehicle). 6 speed transmissions 12/18 on the mileage.
2010 - New iDrive, same 6 speed transmission.
2011-2012 - New iDrive, 8 speed transmission. Mileage improves to 14/20*
2013 - LCI (life cycle impulse aka refresh) - just out. Has M sport package with extra HP optional. Note the extra HP was also available as a factory and/or dealer option on the 2010 and as a dealer option on the 2008-2009.
That should get you going
There is a thread or two here on must have options as well. Your local BMW dealer can run the VIN for you and tell you what exact options a car has (say you are buying it from a non-bmw dealer).
*While the mileage is supposedly improved, I still don't get 14 in the city and the 8 speed auto either starts out like a turtle or slams you back in your seat. Based on this, I personally think the 2010 is a great used one to get - especially with the extra HP