Column Address Strobe (CAS) latency, or CL, is the delay time between the moment a memory controller tells the memory module to access a particular memory column on a RAM memory module, and the moment the data from the given array location is available on the module's output pins. In general, the lower the CAS latency, the better.
So it can be either. The lower the numbers, the faster the memory. As memory bandwidth increases CAS number go up. Fast DDR2 800MHZ memory is 5-5-5. Fast DDR3 1600MHZ memory is 8-8-8.
The faster the memory, the better the build quality and the better the rep of the company, the more expensive the memory is. Which is why you’ll pay more for basic Kingston DDR3 than for Mushkin or Gskill premium DDR3.
So, get memory with a low CL number, with an attached heat spreader that is in your price range. Also be mindful of the number of slots on your motherboard and get sticks that will combine to the amount of memory you need.