I've gone over this scenario a dozen times in my mind, searching for pros and cons. In the end, which is best is a judgement call. Just having ANY sort of backup is a ZILLION times better than none at all, so I find it hard to argue very hard with anybody about backup details.
I don't believe that Redundant RAID is an cost-effective setup for an external backup system. I think the money spent for the second (mirror) drive would be better spent on a SECOND backup drive.
If you have the money for a second (or more) hard drive, I'd use it to swap out the backup drives periodically, ideally keeping one set offsite. (DEFINITELY if the data is highly valuable).
Mirrored or not, periodically test your backups. Waking up to a failed computer and then finding your backups are worthless, too, isn't good. Partially for this reason I'd rather have two independent sets of backups from different times than a single mirrored backup. Also, by definition, a mirrored set is more complex than a single drive, raising the likelyhood of a failure of some sort by non-disk causes. Finally, a RAIDed backup set isn't going to help if a worm or virus erases all the drives attached to the PC. That's where the offsite (or at least disconnected) backup drive comes in.
YMMV. There are certainly pros and cons to both approaches. But it'd be REALLY hard to lose everything with two independent backup disks with one being kept offsite.
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