SATA 6Gb/s (please refrain from using non-existent SATA III/II) would offer better cache burst speed to current generation HDD, as some latest ones are now upto 64MB. That's just from a technical POV. In real world it would be utilised differently depending on the circumstance.
In current state of NAND density with most based on 2-bit per cell, it takes at least a 128GB SSD with 8 internal channels or more to saturate 300MB/s so for the upcoming generation of 4-bit/cell NANDs the arrival of SAS/SATA 6Gb/s is quite important. Nothing to worry about with what's available on market of course. But the time you need it it'll be another 1.5-2yrs down the road.