Both mobos are good, but if you want to benefit from PCie 4.0, which means mainly memory devices and maybe nVidia's Ampere cards in the future, you have to take the newer one. With the new mobo, you also have the advantage that you can upgrade the new Zen 3 processors, which will be released in September, without much effort. The VRMs on the b450 are also only just enough for the r5 3600, you should be able to OC it, but you won't be able to expect a heavy OC with another CPU, as the board only delivers around 75A safely, at 100-125A you'll need quite a bit of airflow.