No difference. SATA, SATA II, SATA III all use the same cable and headers/ports. That is what 'backward compatible' means. SATA is still SATA. The newer versions simply improve on the throughput.
In the case of mechanical HDDs, it won't make any difference in speed because HDDs cannot even match the bandwidth of SATA II, let alone SATA III. SATA III was designed as a faster interface for SSDs.