To know the difference, you have to know what CL means. The CAS latency is the time between accessing memory and being able to read the memory from the output, so 8 is faster than 9 is faster than 10. BUT and this is a huge BUT. Notice the all caps. Sometimes a higher CAS is faster because the memory operates at different speeds. For instance a DDR3 1333 module runs at 666mhz and ha a cycle time of 1.5ns. With a CL of 6 that means it takes 9ns for the memory to be available on the output. However a DDR3 1600 module runs at 800 and has a cycle time 1.25ns. With a CL of 7 it takes 8.75ns. As you can see, even though the CL is higher, it is still faster because the mhz jump increases the cycle time enough to give it a little boost. Going further, DDR3 2000 is just as fast at CL9 as the 1333 is at CL6.
You're not going to notice anything different. the only time you would notice, is if you are running one of AMD's new APU processors. It was shown that faster memory did speed up the gaming experience for the integrated GPU.
Better to go for 1600mhz cl9.