4TB disks on ReadyNas Ultra 6?

Hi - dont mean to Hijack this thread - but
mgdm seems to be very knowledgeable (already answered a question of mine)

and the recommendation seems to be RAID-6 or X-RAID2 dual-redundancy with NAS drives

Ive managed to pick up TWO ULTRA 6's cheaply. (and I should note I have a High availability back ground but in software - not at this level so im used to putting together systems that have to work no matter what -but when it comes to hardware / storage - I just leave that to others - my point being - I know that I over design things and its because of decades of doing it that way for work - i cant help it. I even have dual ethernet cables running through the house sigh - anyway)

So I have TWO ULTRA 6 NAS's - and I plan to put in 6 X 4 TB Seagate NAS drives (based on feed back Ive already got - thanks to all).
So my question is given I have two - do I
Run two separate NAS's - with RAID-6 or X-RAID-2 giving me a total of 32TB of storage. (My hesitation is what if the NAS itself dies)
OR should I run two Mirror'd NAS's - RAID-5 - giving me a total of 20 TB High redundancy ( that way if One NAS itself fails I cna get it fixed while the other one is running - even if i shut it down to reduce the chance of it failng)
OR should I run two Mirror's NAS's No RAID - giving me 24 TB of space redundant as of the last mirror (nightly)

What Im not sure about is WHAT DO you do if the NAS itself dies ? has anyone had that happen (never owned a NAS before - just used Mirrored Disc's at home)
What are people's thoughts and/or experiences on this issue ? should I post this elsewhere ?
Thanks