Probably just a personal thing but I do not like Flash drives, at least not the one I have, probably because it is very unreliable, it fails to store files more often than it ever succeeds, with a error message of "corrupt data" etc when trying to access them later. (one of the very early models)
However, one of the main points to look out for with either Flash drives or Memory cards is the "speed" (access speed).
The faster the card/drive the dearer it will be.
Also the faster the card, the quicker your camera would be, ready to take the next shot or when viewing shots (or files).
I purchased a 320 GB External USB HDD from PC World last year for £79.99 and find it pretty good, although USB equipment will always be slightly slower than a internal drive.
But, the biggest draw-back with "large" USB drives is that they need a separate power supply and I keep forgetting to switch mine off when not required.
Most smaller USB drives (GB xx?) do not need a separate power supply as they draw power through the USB socket, therefore automatically switching off when the PC is powered down.