Those numbers describe different latencies that are incurred during memory access. The larger the numbers, the slower the memory.
Here are the gory details:
Besides brand, the primary difference between your two choices is 1.5V versus 1.65V. This can be a real issue if your motherboard doesn't support one or the other.
Always check the motherboard manufacturer's memory support list. If your motherboard is new, you should be able to find memory that is guaranteed to work on your system. Older motherboards can be hard to find matches for given the short half-life of memory products.
At the very least, see if the preponderance of large DDR3 on the supported list is 1.5V or 1.65V.
In addition, since you are installing a very large amount of memory, verify that your Power Supply is of high quality and sufficient amperage on the system board side rails. Most power supplies become a bit unstable (large ripple and voltage variances) when taxed at their maximum on one rail. If you have two CPU's as well, your system may not be well supported by a gaming power supply that expects the big loads to be on the peripheral rails (dual video cards, RAID, etc.).