1333 is the stock speed, not the speed it's always at, 1600 is an OC speed ie not stock. 1600 cl9 is faster and is what the ram to cpu connection will be at if you set it to that. You do see a small difference, but everyone says it's not worth it because on SB the ram speed is independent of cpu speed when overclocking (and partly unnoticeable because normal peoples' tasks). http://www.bit-tech.net/hardware/memory/2011/01/11/the-best-memory-for-sandy-bridge/
This even shows benchmarks comparing different speeds with different cpus: SB, lynnfield, and nehalem. And you can see with either, ram speed is only a small difference.
Everyone recommends 1600 because the price difference is currently only $1-$3. And there are a few minor performance drops with 1333 as seen in the benchmark.