I want to put the hard drive (500GB) that came with my MacBook Pro (13" mid 2012) in an external (USB 3.0) hard drive enclosure. Are there any benefits to choosing one with the SATA III (6Gb/s) interface for this hard drive, or would SATA II (3Gb/s) work just as well?
SATA II enclosure will work just fine. A benefit of getting the SATA III is that it will be compatible with drives you install at a later date. So if the price difference is negligible then go for III.