Does not really matter to be honest, just ensure you use 2 sticks as dual channel makes a large difference in game and application performance. I have 3200 RAM and set it to the required profile in the motherboard. If overclocking the RAM is important then the newer X570 boards maybe better due to enhanced cooling.
I might even consider 32GB if I was building new as RAM prices have fallen a lot (I paid £110 for 16GB in December)
There are a lot of great deals for DDR4 CL16 3000 / 3200MHz at the moment. Do a search over at HUKD and you'll see what I mean. I posted one up in the Zen 2 thread, 16Gb 3200 CL16 for £56 (alas, it's expired). I'm probably going for the 3700X as well, and 3200 looked the sweet spot. From the recent tests, it doesn't look as though OCing beyond this will yield a significant improvement.
Unless it's changed with these new boards, RAM is the industries dirty little secret. When I looked into this about 3 or 4 years ago there was almost no difference between the top and bottom end of ram based on frequency as far as real world speed was concerned. On paper it looks great but the reality is in use you'll barely see any difference if any at all.
CAS latency is a slightly different matter and ideally lower is better (and faster) and that can make a few FPS difference. But the reality is, for most users and most systems you'll not really notice much difference between top and bottom and certainly not for the price difference.
So my advice (fwiw) is go for 32Gb of the cheapest lowest CL ram you can find but even then I wouldn't pay more than £20 for faster CL. It's just not worth it imo.