Afa the speed goes, try the USB 2.0 cable and see how it goes. I have a 500 GB Seagate USB 2.0 external. I bought a couple of SIIG USB 3.0 cards and USB 3.0 docking stations. While I was waiting for the docks to arrive I plugged the Seagate into the SIIG. I got at least a 15% increase in sequential read/write speeds. Copying a GB file on was consistently 24 MB/s when plugged into a USB 2.0 port. Plugged into the SIIG I was getting transfers consistently in the 28 to 31 MB/s range.
But USB 3.0 tends to vary quite a bit. I've seen a lot of people with USB 3.0 native to the machine, complain they weren't very fast. You might get lucky like I did though.