PC5400 memory with cas5 should match PC4200 with cas4 timings when running in 533mhz mode. Remember your PC5400ram will work at PC4200 speed with the fastest latencies available in this case it shall be 4-4-4-12 or something like this.
I think you have the question a little off. My current Ram is: PC4200 CL-5.
The two choices to BUY are:
PC 4200 CL-4
What is better to match since I can only match one, the CL or the speed?