Well since noone answered you ill try to asnwer
Okay you got fair access and preferred access
In fair access each clien get the same amount of air time this is good when you need equal access to network in a mix of 802.11g/802.11n regarless their capacibilities...
In preferred access well the faster clients i mean for example n clients do not get penalized by slower clients...For example a 802.11g will get more airtime than a 802.11b.
It is recommended that in high density areas you use fair access, you can read it on High density VRD
http://www.arubanetworks.com/technology/reference-design-guides/
Its a lot of good info in there.. its a lot of reading... i have not read it all yet myself but its really helpful!!
Hope this helps you