RN316, six drives: RAID 5 vs RAID6?

It finally dawned on me that the reason my RN316 came up with RAID5 was that OS6's initialization process makes that decision for the user: 4-6 bays, the user gets RAID5 unless they jump through some hoops within 10 minutes that no normal first-time user would know about.

I've been all spun up on the assumption that, somehow, I need RAID6.

I chose RAID6 for my Ultra-6 just on GPs: having the array survive 2 drives failing seemed better that just one drive failing .... and the space was sufficient for my needs.... No actual knowledge or experience whatsoever was involved in that decision: it was strictly from the gut.

But with OS6 choosing RAID5, it's starting to dawn on me that just maybe some smart people somewhere decided that 5 was appropriate for 4-6 drives.

Now that I have two boxes: the old Ultra-6 and the new RN316 - with the Ultra-6 destined to run OS-6 and  become the backup box for the RN316, I'm thinking that I would already have 2-drive redundancy in that each box could tolerate the loss of one drive, and what're the odds of both boxes losing a second drive before I can get a replacement delivered and synch'd (which would seem to be about four days max)?

So I'm thinking: "Why try to fool Mother Nature?... Just keep the RN316 as RAID5".

Once the 316 settles down and I become confident in it, I will replace the Ultra-6's OS with OS6, let OS6 do it's thing with setting up the array as RAID5 and everybody should be happy....

Do People Who Know see any flaws in this line of thought?  

If you had just these two boxes: would you feel comfortable with both of them being RAID5?

(FWIW, I also have a slightly-offsite (in the garden shed) DriveBender box that is air-gapped from my LAN except when I'm running a weekly-monthly-depending-on-when-I-think-of-it mirror backups.  ... It's no prize, but it does stumble along and makes me feel a little better vis-a-vis a fire or destructive burglery.)