X399 is quad channel, you get a benefit from using 4 dimms, not from using 8. With dual channel systems it´s the same 2 is better actually. Having more sticks is harder on the memory controller thus it´s harder to reach the same OCs sometimes. But on the same speed and timings there should not be a difference between 2 or 4 sticks in a dual channel system (or 4 and 8 in a quad channel system). At least not to my knownledge. Assuming they can run at the same speeds and timings in every configuration.
But looking at your RAM choice you get the benefit of more RGB I suppose. 