Honestly for technical users the RPi should be preferred IMO. The reason I want to get Android working is to bring self-hosting to the masses. Turning an old Android phone into a personal cloud by installing a couple apps and putting it in a corner would be huge.
Android is such a pain we might have to settle for shipping custom SD cards for RPi's though.
The ideal thing would be if you have multiple phones and can store one offsite at a friend's house. But that requires more complicated software and assumes people have multiple old Android phones laying around. I think more likely you'd pay a cloud service to handle backups for you. You just need to provide them with a read-only key then they can access the same way you do.
Android is such a pain we might have to settle for shipping custom SD cards for RPi's though.