Should I get a SATA 3Gb/s or 6Gb/s hard drive for my MBP?
As a conventional hard drive wouldn't come close to 3GB/s, I say save some money and go for the cheaper 3GB/s one. If you have money to splash then instead I suggest you go for a SSD, because that would really lift your MBP's performance. It doesn't have to be as large and your current drive, because you can use your current drive as a external one by getting a SATA to USB casing somewhere to take some of the load off.