At this time, the U600 USB Stick for USB 2.0 is the only USB stick device that Virgin Mobile offers. The other devices (Mingle and Overdrive) can also use USB 2.0 via the data cable.
USB 2.0 versus 3.0 is not really a bottleneck. Data transfer for USB 2.0 is rated as 480 mbps from the USB device to computer. You should be more concerned with the data network. The U600 and Overdrive Pro use Sprint's 4G WiMAX and 3G data networks. The Netgear Mingle uses Sprint's 4G LTE and 3G data networks.
3G, 4G WiMAX, and 4G LTE indicate which data network is used for web access. Each network has different maximum speed capabilities. Both 4G networks are much faster than 3G but neither 4G network is as widespread as 3G at this time.
Virgin Mobile answered your question in the FAQs:
• Data speeds for 3G devices on the 3G Nationwide Sprint Network range from 600 kbps – 1.4 mbps*.
• Data speeds for 4G devices operating on the Sprint 4G (WiMAX) Network, typical speeds range from 3 – 6 mbps*. The Sprint 4G (WiMAX) Network reaches over 70 markets in the US.
• When in a 4G LTE coverage area, customers may realize average download speeds of 6-8 Mbps, upload speeds of 2-3 Mbps and peak speeds of 25 Mbps*. This still isn't as fast as the USB connection from device to computer!
*Actual speeds may vary based upon plan or other factors.
• Check your COVERAGE on the top line of this webpage.
Follow the link at this FAQ: http://virginmobileusa.custhelp.com/app/answers/detail/a_id/3164/kw/coverage
Be sure to select your coverage interest such as 4G LTE, 4G WiMAX, 3G Data Coverage, or Voice Coverage in the boxes above the map.