Ever wonder about the memory comparison between CAS 8 and CAS 9 and how much difference in performance that CAS ratings give you. Here is a comparison that I made recently when I upgraded my RAM. This was done from the same computer with no other changes other than replacing the RAM. The memory capacity and speed (1600mhz) where the same.
CAS 9 (GSkill Ares)
CAS 8 (Crucial Ballistix)
As a rule, you always the fastest memory possible but as you can see from the charts, the performance is not significant and is marginal at best. Since the price difference between the two can range from the same to a hefty premium, I would let the pricing determine which you buy. If you can get CAS 8 for the same prices as CAS 9 or within $10, I would go with the CAS8, however if it is more than that, I would not bother as you will not be giving much in the way of performance.