it depends on what you are doing. For most people 1333 is perfectly fine. Over clockers like 1600. The 2133 and 2400 are for extreme enthusiasts, and you will likely not notice the difference between it and 1333 unless you know what you are doing.
Paying extra for high speed memory is in general not worth it except for people that are trying to do record breaking overclocks. In most cases the speed difference would not be noticeable but measurable with benchmarks.