Look through the last few pages of the MBP forums... there are various threads with the same question you have.
I'm not trying to be rude, I'm just saving me and the rest of the people on here from having to restate everything that has already been said, and also save you time waiting for an answer
There really is no answer to your question, it all comes down to what you prefer and what your machine supports. The new MBP's support SATA III, but whether or not it works without freezing up every once in a while during use has been hit or miss. A lot of it is brand specific, others it is completely random. Many people have had absolutely no problems with SATA III, many have had so many problems they revert back to a SATA II drive or their stock HDD.
Please understand that SATA III drives do not always mean faster. Full SATA III speed means 6 GB/S TRANSFER speed, not read/write speed of the drive itself. Therefore, a SATA II drive may be faster than a SATA III drive if its speed specs are better.
As I have said before, do not let speed take the back seat to reliability. SATA II will probably be the best pick if you are not working with huge files all the time (which is where the benefit of SATA III comes in). SATA III also is a more expensive option... and you are most likely not going to notice any speed difference through normal every day use. Trust me.