you cannot compare them.
like asking what is better a KM or a mile. they are different measurements altogether.
CAS is measured in the clock frequency, increase the speed and you MUST increase the CAS.
you can only compare CAS to RAM with the same speed. for 3200 14 is better than 16, for 3600 16 is better than 18, you notice how the numbers rise together.
my old DDR3 1600 had a CAS of 9 because of the low clock speeds (800Mhz).
faster RAM MUST have higher CAS speeds.
if you are getting a ryzen 3xxx series CPU 3600 should be your lowest speed RAM considered, higher if the motherboard will allow.
in whichever speed you decide on get the lowest CAS you can available for that speed.