In my experience, HDDs fail more often than SSDs, but when HDDs fail they usually do so gracefully, losing just a few sectors at first. When SSDs fail, it is usually catastrophically.

Additionally, my guess is that HDDs are probably less likely to fail if sitting on a shelf, unpowered, for a long time, than if they're powered up and running. But SSDs might be more likely to fail if sitting on a shelf, unpowered, for a long time.