cl-10 vs cl-9

Im gonna upgrade my RAM from 4GB to 8GB i have 4 ram slots on my pc and my ram is a 2gb+2gb i'm going to buy another 2+2 but there was 2models one ddr3 1333MHz CL-10 for $8 and another ddr3 1333MHz CL-9 for $10 so which one should i buy and how much difference between the cl-10 and cl-9