USB is slower then SATA. The new SATA 3 standard operates at a theoretical maximum rate of 6.0 Gbps, while USB 3 operates at a theoretical maximum of 4.8 Gbps bi-directional. If we crunch these numbers…
USB 3.0 SATA 3 4800Mbps 6000Mbps each way 600 MBps 750 MBps each way USB 2.0 SATA 2 480Mbps divided by 2 3000Mbps each way 240Mbps or 30MBps 375 MBps each way
EDIT: As per @MrAlpha, I stand corrected on USB 3; it can take advantage of Full Duplex (as described here)
The enclosure lacks the faster UASP mode. Real world throughput of USB 3.0 has a maximum of 400MB/sec (USB 3.0 has 36% overhead in the best case scenario).
Typical USB 3.0 devices seem to be only BOTS enabled limiting them to a maximum of 250MB/sec although in my case this seems to be a slower drive enclosure.
eSata 3.0gbps is still faster than USB 3.0 in the average case at 300MB/sec real world. Disappointing.