Not much. rsnapshot is based on rsync and hardlinks. So, you'll get some whole-file deduplication for unmodified files. No compression, no encryption/authentication, no way to verify if a backup is still ok.

Also, if you rename a file or folder, that hardlink-based dedup will not work, because it is based on files having the same path. Also, if you change just 1 bit in a huge file, it will add the whole new file because the dedup only works on whole files.

borg does variable size chunk based deduplication. if a big file changed just a bit, your backup repo will also just grow a bit when backing up that file.

also you get compression, encryption, authentication and the ability to check backups if they are ok still.

also, borg won't create a gazillion of files/hardlinks in the destination filesystem, but way fewer files (each about 500MiB big by default).

borg also archives ACLs, xattrs, flags.