Two main differences:
- The Studio II model is using 2 2TB drives linked in a RAID 0 configuration to get you 4TB of storage. The Studio model is using a single 4TB drive.
- The Studio II has multiple interfaces: USB 2.0, Firewire and eSata. The Studio model is USB 3.0 only.
To me, each has their strengths and weaknesses depending on your situation. Personally, I would (and actually have) gotten the USB 3.0-only model, but that's because all my Macs have USB 3.0 and can take advantage of the much faster speed. Also, I don't particularly trust external storage that uses more than one drive, and RAID 0: more drives with no redundancy equals more parts that can fail.
On the other hand, for your situation, the Studio II model might be a better choice if you're willing to spend the extra cash. If your Mac has a FireWire 800 port, then that will be faster for you than going through USB, since your Mac has only USB 2.0.