Dumping the data is cleaner and faster. Then add in the new data when they submit the form. If you're worried about something happening between deleting and adding as @djot suggested, then use a transaction.
http://dev.mysql.com/doc/refman/5.0/en/commit.html
I have done this many times with no issues. It takes all the logic of checking each entry against the database out of play. If there is no reason to check, then just delete, way more efficient.