yes, lower CAS is better, but as you go from ddr to ddr2 to ddr3 the timings will go up higher.
Latency is the number of clock cycles it takes for the Cas to refresh. More cycles=longer time before grabbing more info. But this only counts when comparing apples to apples (meaning one speed to a stick with the same speed). Many people will overclock their RAM at a higher frequency, but need to loosen the Cas latencies a little higher to do so. This is typically more beneficial than a slower speed with a lower latency.
so if you want to overclock alot a default higher cas is better so you can bump it up a lil to accommodate the higher clock speed but if your a type of peson who dosent like that stuff then the lower one is good