There is a critical element that should be considered in these SD vs CF primary discussions. If you use a single CF card as backup, you will be writing to it every time you shoot, then reformatting it when you successfully off load the images from the SD. That much writing and reformatting will affect the life of the CF card, and it will eventually fail.
I am not sure if an SD would follow the same failure path. I would guess SD cards also have a limited use life.