So, do you need speed, or just size?
What you describe (mirroring two stripe sets) is RAID 0+1, if you stripe two mirror sets then you get RAID 10. There’s a subtle but important difference, as RAID 10 can potentially survive two drive failures if the drives are on separate mirror sets.
Either way, you still end up with something managing the array, whether it’s software or hardware based.
The risk in OS-based RAID is that if the OS gets hosed, it’s trickier to restore the array (at least in Windows, though it’s a lot easier now than it was in the NT days).
I used to have a 12-drive RAID-5 array. It was fast. I used it for video editing.
But it was a pain to expand, because my RAID controller didn’t support online expansion, so I had to migrate data off, then back on in order to increase the volume size.
Now that I don’t do much video editing, I don’t need the speed, so I chose Drobo for its ease-of-expansion and semi-portability.
If your plan is to use the RAID for backup, then I’d recommend RAID 5 (equivalent to Drobo single-disk redundancy), or RAID 6 (equivalent to Drobo S dual-disk redundancy).
Maybe storage density/price-per-TB isn’t as big an issue for you as it is for me because you have less data than I do, heh.