In spite of the extra costs are flash disk any more reliable than cd/dvd?
From what I've read, what makes an SSD drives wear out is constant writing and then rewriting -- like a Windows installation does during maintenance. If you only write to the drive once, as for permanent storage, you don't have that problem. Current generation SSD's use flash memory, which supposedly is higher quality in SSD's than the flash that is used in inexpensive thumb drives.
My thinking at this point is the external drives are likely the better way to go, however I'm not certain if in the long run they are much more reliable.
It's certainly a lot easier to store large quantities of information on an external drive than to burn 50 DVD+R's, whether you're using an SSD or a regular hard drive.
If you use regular magnetic hard drives, you might consider occasionally running SpinRite on them for maintenance, to make sure the drive stays in good shape.
https://www.grc.com/sr/spinrite.htmMost of this stuff you can live without financial records and the like are also a different story.
Actually I print out financial records on paper if they're important. They take up space, but there really aren't that many that have to be saved. Think about how long paper lasts -- even cheap paper like newsprint will outlast digital. And unlike a photograph, it doesn't matter if the paper or colors fade a little.