it's a matter of opinion between both Seagate and WD to be very honest. WD was bleh a good few years back, but they started to come on top these last years. Seagate were pretty much bullet-proof, but then a batch of them a few years ago came with faulty firmwares that bricked the entire HDD for no apparent reason (physically, the plates were fine, it was the controller that went b0rked). This however, seems to be in the past and they seem to step up nowadays.
personal experience from WD which I have now is, I have a 1tb green drive and a 1tb black drive. both awesome and work quite well. The green one is working as storage while the black is the main one. it is of note, however, due to the design itself of the hdd, WD Caviar green drives aren't that great for portable, NAS of to be used as an OS drive. they have some weird power saving techniques (constant spin-down/up from 5400 to 7200 and back) that is time and load sensitive. It is, nevertheless a very performing green drive, and quite the saver on power.
WD caviar black drives are good drives. work well, and are fast for a mechanical drive. better than this only really either a seagate momentus XT (hybrid drive), a velociraptor with it's blistering 10k RPM's or a SSD.