MacBook – SATA II or III for external hard drive enclosure

Short answer: it doesn't matter.

Longer version: Either will work fine. SATA is backwards compatible, so SATA 6Gb/s drives will work on SATA 3Gb/s interfaces and vice-versa. The 2011 Macbook Pros have 6Gb/s ports for the HD (some in the optical bay as well). However, no mechanical hard drive can come close to maxing out SATA 3Gb/s speed, so you'll see no difference from a 6Gb/s drive or interface. It makes a difference with SSDs, but with hard drives, it's merely marketing.

Panther Point has 6GBit/s ports. There is zero reason for Apple not to utilize them. I know this is not a perfect answer but this is all I can help.

SSD is much faster on USB 3.0 than a HDD. I would recommend reading this -- disregard the UASP part, it still gives you lot of benchmarks of running an SSD over USB 3.0. You can clearly see speeds well above 200 MB/s. There is no mechanical (notebook) drive that can do much more than 100 MB/s. Not the Savvio, not the Spinpoint M8, nothing. It simply doesn't and cannot happen.

Edit: StorageReview mentions this Hitachi DeskStar being the fastest mechanical drive of all and it barely clears 150 MB/s.