Im currently in the process of putting together my first home built PC and had a question regarding the difference between these 2 drives. as far as I can see, the specs on the 2 are practically identical except the FAEX is a SATA6 whereas the FASS is a SATA3. However, despite the fact that the FAEX has the potentially faster connection (I know most HDs cant even keep up with SATA3), its priced about $40 LESS than the FASS
I don't know if there's a performance difference, but you can get either - they are both high performance drives. Caviar Black is WD's performance hard drive line (just as Green is their low-power / eco-friendly line).
I think the FAEX is newer technology, and cheaper because of it. I think it might have fewer platters (and is therefore cheaper to manufacture).