Cost differentce is primarily in the extened warrantee for the Blacks (5 yrs vs only 2 yrs for the green)
User satisfaction a little higher for the Black. Green = 16 % 1/2 eggratings vs only 13% for Blacks.
As far as the "cache" size - NOT that significant for sustaned through put. It only holds 32 or 64 MB, And that is a "Guess" of what is going to be called next. If File that is being read, or written, is large - then NOT difference.
I normally do NOT recommend the green, blue, or yellow with red pokadot WD drives. My current recommendations are for the Samsung F3 and the WD Blacks.
Samsung F3 500 gig ($60 and 11% 1/2 egg ratings)
http://www.newegg.com/Product/Product.aspx?Item=N82E16822152181
Newegg did have the 1TB Samsung F3's onsale for $80 - Bought two last week.
Ref: on WD drives:
Black
http://www.newegg.com/Product/Product.aspx?Item=N82E16822136795&SortField=0&SummaryType=0&PageSize=10&SelectedRating=-1&VideoOnlyMark=False&IsFeedbackTab=true#scrollFullInfo
Green
http://www.newegg.com/Product/Product.aspx?Item=N82E16822136358
Added - Diff on 640 (cheaper) vs more expensive 500 Gig drive.
My guess - 640 is an older drive that uses to platters (density = 320 per platter) while the 500 gig drive uses newer single platter (500 gig/platter)