the value RAM is targetted at the mainstream user. it'll run 100% stable, at default specs.
the difference is that corsair's XMS series is targetted at enthusiasts. they take pride in the speed and stability for all products, but this is the top of the pyramid. the timings of the RAM are much tighter, especially important for AMD based systems, and the RAM is designed to be tortured by overclocking. you're paying for the better memory chips, that extra little detail that counts for overclocking.