SxS (S-by-S) is a Flash memory standard compliant to the Sony -created Express Card standard. According to Sandisk and Sony, the cards have transfer rates of 800 Mbit/s and burst transfer rate of up to 2.5 Gb ps.] over the ExpressCard’s PCI express interface. Sony uses these cards as the storage medium for their XDCAM EX line of professional video cameras.
I have found the PMW 100 XD CAM to yield very high quality video due to the higher bit rate the camera is capable of recording.