The difference is obvious when you chain multiple SATA drives into a RAID configuration. If you have four standard hard disks in a RAID 0 configuration, the array would peak at 240Mbps, which would be faster than the SATA 150 disk drive. The SATA 300 would be better in that configuration. If you have just one or two hard drives connected with a SATA disk drive, you probably won't see any difference. If you have three or more hard drives connected, the SATA 300 would allow the drives to realize their full speed.