Based on hardware differences, only the PUH (pickup head) is different. Firmware seems to be similar in quality thus far. They are pretty much equally good if you do not plan to overclock the drive.
Based on overclocking methods the 451S is somewhat easier to overclock (to a 851S) and early indicators point that eventually we may be able to overclock it to 8x -R easier as well. As with any overclock though, no promises of course.