You have to start with the assumption that if a storage device fails, you won't ever/economically get any/trusworthy data back off it.
From that starting point, you ought to have enough paranoia to assume the worst, so you begin with the question of what happens when (not if) your device fails/corrupts?
RAID save you down-time, both use (machine keeps working) and admin (no need to restore your backup) but RAID!=Backup as we are always told.
Also most RAID & file systems don't have integrity checks so you can have data corruption and not know until something starts playing up. Once you realise this and the vast amount of data you may need to store (comparable to the 10^14 bits of HDD error rate) you might want that, so you then invest in ECC memory and a file system like ZFS or GPFS that has checks. They also support snapshots, a vastly under-rated feature that can save a lot of hassle in restoring a just deleted/modified file, or simplifying a consistent backup point-in-time.
And there there is your backup, which ought to be in another building and not on-line as a mounted file system or you might get randsomeware screwed (something that snapshots can also help with, if you notice soon enough).
Really the arguments for SSD vs HDD that matter are cost/GB and IOPS, and smarter systems will use both to give to lots of storage at good price and responsiveness.