Theoretically, X370 or B350 shouldn't make much of a different, but in reality, motherboard makers see B350 motherboards as the budget options and put much better components (i.e. VRM) on their X370 motherboards.
If chipset is the only difference, I would absolutely go with a B350 motherboard.
Another important factor is that the processor is expensive to upgrade while memory is cheap(er).
Ryzen 7 1700 is going to be a dud. (the bottom of the barrel)
Think about it, if it wasn't, it would be a Ryzen 7 1800X or a Ryzen 7 1700X.