..When buying a seperate enclosure and a Hard drive to go into it, I look for both esata and USB3. For HDDs esata is slightly faster than USB3 even though spec is higher for USB3 (native vs translation).
.. Most of the newer compact BU HDDs do not sport both interfaces.
.. Usb3 will be more portable (ie going from a Desktop -> laptop)
.. For esata and desktop, would want a front panel estata - easiier to connect/disconnect.
.. esata requires seperate power I think, while USB3 provides more power (Current capability) than USB2 which ielimiates external power for a drive over 2.5 W.
As stated earlier, USB3 is backwards compatable with USB2 which also improves porability.