I use the SD as primary and the CF as overflow. Main reason is the computer I use has a built in SD card reader.
Question: Which is actually faster? Why would I want to use the CF as primary or the SD a primary?
As for backups, yes, I have had cards fail but not so bad that I could not recover the card. I try to keep an eye on what is going on during a shoot so if there looks like an issue, I select the CF slot and/or insert a new card. Most of the failures I have had are due to the card not being inserted correctly or not powering the camera off before removing the card.