Another decision point may be performance. I have read some comments on this forum about dual disk redundancy (DDR) reducing either read and/or write rates (I think/hope writes only?) but I have not refreshed my memory lately as to the extent.
Personally I think it is a huge mistake to rely on the Drobo - or ANY other disk array - as the only copy of my data. There are too many things that can go wrong and many of those things are totally outside the control of the Drobo. So nothing I am saying is a slam against the Drobo. A Drobo may very well be the most reliable disk array available- I don’t know and can’t have an educated opinion on that. My experience is great over 2.5 years with a V2 but I never had a disk actually fail out of the array. I do burn my disks in before installing them in a Drobo and I have lost a new one during burn in.
(I would not even rely solely on professionally managed internet cloud backup services- there have been a few very publicly discussed failures and loss of data already)
I have seen enough reports here of failed Drobo arrays - often starting with a single disk failure that ends badly - to assume that losing 2 drives is common enough to be cause for concern. And with an S the odds must increase with a 5th drive that is a candidate to fail during a rebuild. Instead of 3 chances for a 2nd failure you now have 4.
I keep multiple backups of everything stored on my Drobo. I very recently ran into an opportunity to acquire an S (V1 USB 2.0 version). Although I do back it up it is not current every minute of the day and given the slowdown in performance during a rebuild I don’t think that is the right time to try to force an upgrade of backups. Plus I occasionally travel with my Drobo and then I can’t be backed up to the minute and really rely on it. So even for me there is some value in a little extra margin of safety with dual disk redundancy and I may pop for a drive to do that. My main concern is the performance issue because I don’t have a pressing need to fill the S with 3TB drives yet.
If I didn’t back up my 5 bay Drobo I would absolutely run DDR. If you’re going to roll the dice on these things, it’s best to rig the dice ever so slightly more in your favor.
I understand the OP has thought this through; I added most of the above just for general public consumption.