If you have really important files
Even pictures of loved ones for those seperated are priceless
I would go with either the usb or hard disk with a seperate backup of the really important irreplacable files (pictures, certain docs on cd)
usb flash memory is good small and you dont need a mains or battery supply. dont need defraging etc. Also can take more rough treatment while operating than a HDD.
But can die at a moment notice and have sometimes alimited life span. Also important to think of the transfer rate as can vary a lot due to the make and type.
Hard disk is far bigger and do remember in 1995 most of us were amazed to have a 1gb hdd now programs can be bigger than that. So if you intend to keep for a while best to think maybe not 13 years but it does not take long for techno gadgets to become outdated.
So personally i would go for the HDD.
Now days i use mainly flash hdd which are not big but have the best of both worlds. But then i dont pay for my gadgets