Never use file-level backups of the SVN repository directories. Even if you are using FSVS.
"svnadmin dump" is good for long-term archives because it is platform-neutral. The downside is it takes a lot longer to generate and uses up a lot more space (even compressed with gzip -5). It also fails to backup things like the repository scripts and a few other things.
"svnadmin hotcopy" is our preferred method for nightly backups. With SVN 1.8, the hotcopy process is incremental and you no longer have to delete the target directory prior to the hotcopy. Our nightly hotcopy went from 2-3 hours down to 10-15 minutes after switching to SVN 1.8.
(After we do the hotcopy, we use rdiff-backup and push to a backup server. This has the advantage that we can revert the repository to any day within the last 6 months.)
A third option is svnsync, where you keep a warm-spare SVN server up and running.