I think microSD cards are a bit to small to be convenient. They easily get lost or broken. USB has a nice form factor for storing and transport but the UX for the player is worse, as the stick doesn't vanish in the player like a CD or a microSD card. I think the best UX, would be an SD card the size of a bank card, that can be put in a slot, but the marginal difference to a CD is really low.