Right now is a TERRIBLE time to buy a HDD. main difference between the 2 is that the Black Caviar spins much faster and gets better speeds then the green. I HIGHLY recommend Black Caviar drives (got 2 2TB drives myself)
I have the two WD Green Caviar Drives. One is 1.5TB, the other is 2TB. They playback HD video to a single device fine and could easily a second device. Don't know how many feeds you could record at once though.
The black drives are faster though, so get whatever fits your needs.