Being solid state, SSD’s aren’t prone to the mechanical problems that can happen with HDD’s. The capacitors in SSD’s can lose charge over time. The colder they are, the long they’ll last. HDD’s can have issues with lubrication and crashing if they’ve been sitting around. They’re also much more shock sensitive.
The advantage that the HDD’s have is that they tend to die is increments, showing increasing signs of death. If you’re using them continuously and keeping an eye on them, you can often get enough warning to move the data and replace them. SSD’s will often just stop working one day.
The truth is than neither is great if your plan is to unplug them and put them on she shelf. A better system is one that keeps them powered on and does a data integrity check every once in a while, and notifies you if your need to replace something.
If you want something sitting on the shelf, you’re probably better off with an archival optical disk.