I've been using Kingston DataTraveler Elite G2 32GB flash drives with my Yamaha keyboards.
I wouldn't worry about USB 2 vs. USB 3 as much as a qualified (measured) WRITE speed. Flash memory reads much faster than it writes. This is why people run into problems recording to a flash drive even if it reads audio (or whatever) without error.
If a manufacturer does not explicitly state the write speed, I won't buy it. This often indicates slow, cheap flash memory.
Faster write speed means your PC/Mac can write to the drive faster, too. This is helpful when transferring large amounts of audio and other data.
Hope this info helps -- pj