I personally use 2 ZFS groups of 2 disks, and backup one on the other 'manually' [0]. I get the advantage of both worlds mostly.

I use a HP microstation gen 8, it's lovely hardware; and I use nas4free on a usb stick in the internal port as software.

[0]: by manually I mean the machine that use obnam to backup use one or the other group as a destination, depending on the day.