It does not matter one bit when talking about mechanical hard drives. Not even the 10,000 RPM Velociraptors can saturate the 3Gb/s bandwidth provided by SATA 2. The even faster SATA 3, 6Gb/s standard is for SSDs. Even my 2 year old Vertex 2 SSD only uses the SATA 2, 3Gb/s standard. It still boots in about 15 seconds.
So when you see a hard drive marketed as SATA 3 or SATA 6Gb/s it's only marketing hype. It means nothing as far as drive performance goes.