Only a few ssd's support the new sata standard; some are running at about 375 instead of 275 for the read speed. The new crucial drives run about 100mb/sec faster using the sata 6 standard.
Yes... SATA III SSD is faster than SATA II SSD. On mechanical drives, it doesn't make sense to go with SATA III, as the speeds don't even push the limit of the SATA II throughput.