Ghost 9 is the only one I know of that will recognize the USB drive. (At least that's in the specs.) Other programs like BootIt Next Generation image from DOS and so don't have an easy way to recognize external drives.
As to DVD or USB: USB is certainly faster (and easily rewritable) and with a big enough drive you can put multiple dated backups on it for extra safety but in cases where I have clients with very critical data, I set them up to do an occasional DVD backup and have them take the DVD home (if in an office) or to the saftey deposit box etc. to protect themselves in the event of fire/theft.