After a little research, I see it written that both flash drives and SSD drives are good EXCEPT most USB flash drives have a single channel controller, while most SSD drives have multiple channel controllers. Also, SSDs often have DRAM caches to receive data quicker (since writing to a DRAM is a faster then writing to a flash drive) and generally have SLC caches. All that makes the latter generally faster. On top of all that, SSD drives have wear leveling. Wear leveling is where a drive distributes read/writes around all blocks to prevent some blocks from wearing out faster than others. That is, flash drives wear out faster.
I'm a little surprised that SSD drive manufacturers aren't working overdrive to convince people that their products are better than USB flash drives for reliable, extended, storage!