I buy almost exclusively WD hard drives. Each has their purpose. Here is a quick breakdown among their drives:
Green - Eco friendly drives with design to be energy efficient and quiet. Designed to take advantage of non-use to lower power requirements.
Blue - The "standard" drive - reliability - great for desktop PC use.
Red - Most common use is for NAS - designed to be compatible with NAS enclosures, does not utilize any ECO friendly power reduction, as the drive was designed to run 24/7 in a network environment.
Black - This is the most reliable, highest performing drive in the series. For a consumer based drive, this is the best.
I use the black drives in my HTPC and main rig (Basically doubles as a file server, I have 4 - 2 for backup of other computers, 1 for backup of my computer, 1 for data on my computer). The wife's computer uses blue drives. I am not a big fan of the green drives, as the ECO friendly features can create read/write delays - although they are really good drives. The red drives are basically the blue drive with interface boards that are optimized for NAS and no ECO friendly features.