Digital - easier to carry and to handle while reading. Plus u can read on a phone/laptop in dark environments as well. However for reading on a plane, where I’m sitting in one place, I somehow prefer physical books as they’re less painful than looking at a lit up screen for extended periods of time.
There’s also an option to transfer your chats to another device without backing up to Google drive, but it obviously works only as a transfer tool not as a backup.