I haven't tried either of them but in case you weren't sure - there's minimal motherboard performance difference when it comes to graphics cards. You may see no difference at all, or just 1 fps with the better board. Highly unlikely to see more than that, and that's being generous. More expensive doesn't always mean better at everything, either. Although the chances across the board (pun not intended) are increased.
I remember reading a motherboard performance showdown (they are pretty rare), the reviewer tested a bunch of motherboards for different things with CPU, RAM and GPU at same frequency and there was hardly any difference, although when it came to CPU and RAM overclocking tests some boards performed slightly better than others. Now this is something that CAN affect frames more than the motherboard itself, i.e. the more you can overclock your CPU/RAM the more frames you're going to get. But even then, the actual graphics card will have the far bigger say. If you can clock CPU/RAM a lot on a system with a GTX 1080, you're still not going to get the frames a GTX 1080Ti will give you.
That general observation also needs to be qualified for Ryzen - Ryzen does not overclock a lot, so your chances of one motherboard clocking CPU by more than 100MHz over another motherboard are very slim.
So don't sweat it. You can comfortably go with the B350 unless you need an X370 for some feature the B350 doesn't have.
And I say this often but they still haven't removed this risky feature, so - if you go with MSI don't use their bundled Live Update software, especially not to update the BIOS. Use M-Flash method.