You can't go wrong with either one. Both are great HDD's, and you will get recommendations for both, depending on personal choice.
I like WD not only for their performance, but they have good customer service and you can talk to the tech support anytime.
Given those choices, most folks probably pick their poison depending on the best price or sale at the time.
According to this academic paper all gentlemen who buy Western Digital hard drives are big girl's blouses. It's scientific fact; there is no proof, but it's scientific fact.
It's really down to personal preference and history really. I've used Seagate for years and not have a single drive fail. A month ago I bought a WD20EARS only to have it fail within a week. In truth they are just as likely to fail than one and other so buy whatever you fancy.