The question is why do you think USB drives would be corrupted by magnetci fields. They are not magnetic media, they are flash rom (EEPROM). Flash roms are the kind of thing you put the software on for a satalite or spacecraft where your getting exposed to much more radiation than here on earth.
I use multiple forms since each has advantages and disadvantages.
My flash drive I keep my calendar and address book on and it it automatically backed up each time I plug it in via a simple hotplug script. Those two files are my absolute most important to me so the flash drive is the third form of backup. I backup those files and all my other data to CD's at regular intervals and do a quick dump of everything to an external hdd in between. This way I'm covered for differnt kinds of failures of backup systems.