From my research, an SSD uses flash memory and an SSD controller that allows it to read and write faster, so it will perform well as an operating system drive. Flash memory has a limited number of write cycles. An SSD includes wear-leveling features to retire worn out data blocks after many write cycles and replace them with spare data blocks, to extend operating life.
A USB flash drive uses flash memory with a different type of controller. Its primary purpose is removable storage, small size and portability. It is slower and cheaper than SSD, since performance is not as important for file storage as for an operating system drive. Flash drives do not include wear-leveling, except for some high-end, more expensive models, so they may wear out sooner if they are used for frequent write cycles.
If using a flash drive to backup files, I suggest doing a write+verify to read back and verify the file after writing. TeraCopy includes a file verify feature.
Read the device specifications before you buy.
What is missing to convince me is any real world bench mark data.
This guy runs performance tests all the time, you might want to look over his site. He just got the newest MacBook Pro and likes it a lot, performance-wise. Here are some tests he did with SSDs with the older MacBook Pro...
There are several SSD-related tests scattered in his article index.