Memory quality: not all flash drives are created equal. Especially true with regard to the memory chip that powers your USBs. The easiest way to verify is to work with someone like TigerPress who knows how to verify a supplier. Ensuring tier 1 memory chips are by a reputable manufacturer such as Samsung, Sandisk, Intel, or Micron.
Storage Capacity: USB drives are available in a range of capacities, typically from 64mb to 128gb, cost per drive will increase along with the storage capacity. If your drives will be used exclusively as a promotional handout, we usually recommend opting for at least 2GB in order for the recipient to have enough storage space.
Chip Format: the majority of custom flash drives are offered in USB 2.0 and USB 3.0. USB 3.0 offers increased speed and reliability.
You want to ensure the USB drive you chose matches your branding.