The wider WD MyBook is just 2 x 1TB HDDs. You used the same link for the thinner one, but I'm guessing it's probably 1 x 2TB HDD.
I'm not familiar with either model you mention, though I'm quite happy with my WD MyBook 1TB for off-site storage.
If you haven't already, now would be a good time to consider your overall continuity plans, i.e. redundancy and backup. For example, if you only have your data on a single external HDD...how will you feel when (not if) the entire HDD dies? Will it be okay to lose all your data forever? Will you have a mirrored HDD as a backup, so you still have easy access to all the data? Will you have made diligent optical backups (like DVDs) for access to all the data, albeit with more of a PITA? Will you have an offsite backup in case of a catastrophic event at home that wipes out all your data on all your on-site HDDs?