I have a number of 1 GB thumb drives. I have a 160 GB Western Digital USB drive made for laptops. I have a 100 GB Hitachi laptop drive housed in a Manhattan enclosure.

For me, the trade offs are between physical size and storage size.

The thumb drives (flash memory) are physically small. Easy to carry in a pocket. The only ones of mine that have failed are the ones with the tricky encryption software built in (SanDisk). They failed because the encryption software wasn't all that great. As others have noted, you can get quite a lot of storage on them. If you only need 4 GB max, then double that and get an 8 GB version. You will be happy you did.

8 GB isn't all that much. Most of the decently priced USB HD units for laptops hold 40 GB or more. Seems like overkill. These units are physically larger. I have a Sony Vaio PCG-TR3AP which is very tiny (and old). Sony still makes similar models. But I digress. Even with the small size of my laptop and the modest size of the USB hard drives (slightly bigger than a deck of cards), I still have trouble in First class organizing the laptop, USB cable, and USB hard drive on the fold out trays. No room for coffee. Things are even more cramped for people with normal sized laptops. You just don't have that problem with a thumb drive.