This is a common argument that comes around fairly often in varying guises.
The real thing is to look at your use case and determine what trade offs you are prepared to make.
The real question I would ask is whether more cores or more performance per core is the most important.
Unless you need the 60 PCIe lanes of X399, I might be tempted within that budget to go for a 3700X and X470. Multi core performance is largely similar, but single core performance is massively better.
See comparison
https://www.anandtech.com/bench/product/2126?vs=2520