WD Caviar Black is faster than Seagate Barracuda without any doubt because of its better components and speed.
WD Caviar Blue and Seagate Barracuda performs almost same.
WD Caviar Green is not that much good but it requires less power and WD Green shut downs itself when not in use, this cause to a certian lag sometimes.
WD Caviar Black is much better Hard Drive.