If it is just for storage and backup...
USB2 is fine, it is cheaper and more importantly less problematic.
If you need to actually work on files... you'll need the fastest anyhow.
FW800 is what you will need, you might have problems with OS updates occasionally, but the speed is worth it.
Just research the FW chips used and see which company supports their product with firmware updates. Some out there kick something out the door and never update their product again, making a possible FW brick (happens), though the USB2 interface would likely keep working.