To put it on simple maths:
The frequency is expressed in Hertz, which means "cycles per second". So, the DDR3 2000 will perform 2000 cycles a second while the DDR3 1600 will do, well, 1600.
The CAS latency is given in cycles. So, a CAS9 RAM will take 9 cycles to respond and the CAS6, 6 cycles.
Now putting it together: the DDR3 2000 CAS9 will take 9/2000 seconds, which is equal to 0,0045 seconds, to respond while the DDR3 1600 CAS 6 will take 6/1600, which is equal to 0,0038 seconds, to respond. Thus, the 1600 one is faster.
Less RAM sticks generally means less load to the memory controller. So, 2x4GB is better than 4x2GB.
What a great explanation. Just set out to figure this out today and this pretty much wraps it all up. Thanks, and good for you!