I have a X370-Pro Prime in my home server, paired with a Ryzen 1700. Works like a champ.
To use a second gen Ryzen, you'll have to update the BIOS on the Prime X370-A, to version 3803 or higher, BEFORE, installing the Ryzen 2700x.
With the B450 based motherboard, that is not an issue. You also get a more stylish looking board (if that matters to you) than the Prime.
Both a good choices, to be honest, for most users.