I agree, if you care even slightly about your data, you should buy double the hard drives and have a backup, regardless of manufacturer. But based on my experience, WD and Seagate have two very different failure patterns.
WD drives, if they're going to fail, they tend to within the first couple weeks of use. Failure is rare, but when it does happen, it's usually shortly after installation with WD. If you make it a month with the drive, it's very probably going to last at least the length of its warranty, and probably well beyond. I always buy my drives in pairs, and I always have backups, but with WD, once that first month passes, I've never actually needed the backups, knock on wood.
With Seagate, I've never found any drives to be predictable like that. Failure rate is very high, and any drive can fail at any time. And let me tell you, when a hard drive fails an then your backup fails when you try to restore, you lose any brand loyalty in an instant.