I think a large set of dictionary words are likely more user friendly. I think most people will have a lot more confidence on their ability to transcribe words to/from paper more accurately than a bunch of numbers - better built in error correction, etc.
Sanely formed numbers (like 4 digit groups with a checksum) seems like less writing to me, b/c I hate my hand writing.
> Most people will screenshot it, and those screenshots will end up in unencrypted cloud backups.
At least on Android apps can disable screenshots, though, which might be a simple way to deter people from doing that?