There's another problem with USB3, the host controller device itself has to be capable of handling the increased throughput. This means the CPU/SoC and I/O interface (SATA controller) of the media box has to be able to handle over 35MByte/s to see any benefit from USB3. On low end devices, this is not the case. Not sure about the two devices you describe, but for playback, USB2 should be fine since there is no media type you will be playing at >35MByte/s (only raw video would be there).
Your USB3 drives are backwards compatible with USB2.