Newer Wiis cannot play disc backups. Generally, any Wii Manufactured after the middle of 2008 (aprox.) cannot, as "N" changed the drive. If your console is newer then that, or was repaired by "N" after that time, then it will never be able to, and USB is your only option.
From the peanut gallery, USB loading is easier, as there is no disc swapping, and saves you on discs. Good luck!