Well here's the basics:
Raid 5: If 1 disk fails, you are still ok, so you can swap a spare etc. shouldn't have data loss.
Raid 0: if 1 disk fails, run away, look for a new job, etc. :)
Raid 1: If 1 disk fails, you are still ok, so you can swap a spare etc. shouldn't have data loss.
That said, if you've got 2 identical Linkstation Quads and just want one to be a backup for the other, but want the best balanace between reliability and performance I'd say...
Raid 5 on the primary system.
For the 'backup' system, assuming all it is is a backup for this system, I'd do the same, Raid 5.
While you could get more disk space (and possibly faster backups) by going to Raid 0, doesn't really buy you much as the space wouldn't matter (assuming a 1 to 1 ratio w/ the main system)...so i'd up the reliability and stick w/ Raid 5. No point in adding the risk of Raid 0 if you're not requiring the performance or space additions it offers.
Hope this helps.