RAID 1 with Two LaCie 5Big Thunderbolt
Firstly, RAID shouldn't include your backups. Something catastrophic hitting one, will hit the other. Eg. fire, burglary, etc.

Ideally, you want to keep one set off site. Using the 2 disks you can keep one at a friends, and sync the up periodically. There are tools that allow you to do this. Alternatively, something like BackBlaze, Amazon Glacier, will give you huge or unlimited storage that is remote and protected.

Secondly, when setting up RAID you want to think about your priorities. Are you optimizing for speed, redundancy, etc.

I you want to have one unit in house, and one remote and periodically updated, you're probably better off with RAID5, which is a better mixture of redundancy and speed.

Finally, RAID won't allow you to upgrade disks singly.

Alternatively, using a something like Zevo's ZFS you can configure the Big5 as a JBOD and feed the disks into a ZFS RAIDZ. This will allow you to mix and match disk sizes, and upgrade over time. It will also keep better control of your disks, checksum all the data, and a host of other features.

Personally, I'm a big fan of the ZFS approach, as I think the performance and security are better than a plain RAID.

RAIDing using OSX software, on top of a RAID in the LaCie box is asking for some complex failure.