If you're overclocking, I would recommend getting a better cooler than the stock one. The Cooler Master 212 Evo is cheap, but tried and true in most if not all use cases.
You're better off with the 1600 if you're overclocking. If you overclock, XFR is diabled, so your $50 for the X goes to the trash can.
From a quick googling session it looks like the only benefit from X370 instead of the B350 chipset is SLI and more USB ports. So overclocking looks like it isn't affected.
For aesthetics I would stick with black and blue, since you've already got blue led fans and a black case.
Good luck!