I don´t think you will get usefull answers here. Mostly because there is no correct answer to this.
Apparently both are good companies with good or very good reputations.
My PERSONAL opinion/experience is that I had three Seagate HDDs failing (or starting to fail) after 2 months of usage.
However, WD never disappointed me. So, I won´t be touching Seagate again anytime soon.
WD never disappointed me so far and I will stick with them.
But I bet there are exact opposite stories around.