This really is not a very good question for comparison. A Western Digital Caviar Black or RE4 hard drive is going to be better quality, performance, and reliability than a cheaper Seagate Barracuda drive. Likewise, a Seagate Barracuda XT is going to run circles around and have greater reliability than a Western Digital Green drive.
Each company has product lines which vary greatly depending upon the need and budget. It would be far more viable to ask for input comparing specific product lines instead of the companies themselves because there simply is no answer here.