Does your motherboard/CPU support dual channel RAM or triple channel?
Now, between the CL6 and CL7 G.Skill RAM there probably isn't a huge difference in performance, although the CL6 RAM may overclock slightly better. Even then that shouldn't be a big deal because both sets are rated up to 1600MHz as is, and using the lowest possible memory:bus ratios neither set will hold back your max BCLK/bus speeds.
As for what I'd buy, I can only speak for myself. I started using GSkill memory when they were cheaper than the established brands. Over time and after some disappointments, the prices were the same so I stopped buying them.
You have left ou the most important info, that being what you are doing .... CAS latency impacts will change depending on what you are doing. For my needs, I always buy the lowest CAS I can afford.
1366 based systems
At 6GB (3 x 2GB), the best performance buy is the Mushkin Radioactives. The cheapest set I'd buy would be these CAS 9's from Corsair at $70
At 12GB ya have to decide between 2 sets of the above or one set of 3 x 4GB. The low cost CAS 9 option is $155. Given CAS 8 is the only step up, I'd get two sets of the Radioactive CAS 6 modules for only $25 more.
1155/1156 based systems
At 4GB (2 x 2GB), the best performance buy again is the Mushkin Radioactives. The cheapest set I'd buy would be these CAS 9's from Corsair at $50 ($40 after $10 MIR)
Keep in mind that since you have a Sandy Bridge CPU you need to get RAM with voltages below 1.58v. That is because the Sandy Bridge CPU's Integrated Memory Controller doesn't handle high memory voltages well. There was a story shortly after the Sandy Bridge launch chronicling an alarming number of Sandy Bridge chips dying from excessive CPU voltage, but also dying from RAM voltages that were too high.
With that said your RAM voltages have to be fairly high in order to cause problems, but to be safe I'd stick with modules that can run below the 1.58v level that has been mentioned as "safe".