Recommendations - Drives for Offline Storage

I'm looking into options for drives for 'cold' storage for my institution, possibly for long periods of time (we would like a shelf-life of about four years). They don't need to be particularly fast so much as reliable and long-lived (and, of course, cost-effective). We've already got some LTO tape we use; we're looking more for hard disk drives or solid state drives to be held offline in a climate-controlled area. The scenario involves a write-once, read-a-few-times situation.

So, first of all, does anybody have any experiences or know of any studies about the longevity and data integrity of HDD versus SSD? I know, for example, that SSD is better-equipped for unpowered storage than HDD, but that they can also be more prone to data loss.

Second, does anybody have a particular brand that they use and would particularly recommend to purchase or to avoid?

If you'd rather not answer on-list, you can email me at sarah.barsness@mnhs.org and I'll post your replies to this thread anonymously for you (I could *really* use some very candid recommendations).