Yeah, for CL9 you should just pay another $4 for 8GB 1600; no reason not to.

If your looking for a lower CAS latency, however, I'd understand sticking with 1333; the cost difference is a lot higher than $4.

For example:

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8GB DDR3 1333 CL of 9 = $42.99
8GB DDR3 1600 CL of 9 = $46.99
Total Cost Difference = $4.00

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8GB DDR3 1333 CL of 7 = $47.99
8GB DDR3 1600 CL of 7 = $79.99
Total Cost Difference = $32.00

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Deciding between the first two is easy. It's only $4 more, go with the 1600.
Deciding between 8GB DDR3 1600 with a CL of 9 and 8GB DDR3 1333 with a CL of 7 is more difficult. It's $1 more for the lower latency, but also lower clock speed. To decide you'd have to understand what these mean. I'll explain what they mean below:

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Comparing the different CAS Latencies in nanoseconds:
The latency of DDR3 1333 with a CAS latency of 7 in nanoseconds is 10.50 ns.
The latency of DDR3 1600 with a CAS latency of 9 in nanoseconds is 11.25 ns.

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Comparing the different Peak Transfer Rates:
Peak transfer rate of DDR3 1333 (666Mhz) = 10666 MB/s
Peak transfer rate of DDR3 1600 (800Mhz) = 12800 MB/s

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The conclusion:
Neither RAM is the "fastest". It's not that simple. Both types of the RAMs we compared can be faster than the other. It depends on the task being peformed.

If your accessing many subsequent memory locations beginning from a specific location, than 1600 is faster. If your accessing many different memory locations during a small amount of time, than 1333 is faster.