The B450 mobo if you want assured out of the box compatibility with ryzen 2. If that X370 does not have a BIOS version that supports ryzen 2 a ryzen 2 chip wont work in it. And it doesn't look like there is any way to update the BIOS on that mobo without having a ryzen 1 chip on hand.
So B450 or X470 if you want to be sure the BIOS will be compatible with ryzen 2.