A320's tend to have small under powered vrm's with no cooling and you wouldn't wanna put an 8 core cpu on a board like that so your basically cutting your upgrade path off at a 6 core you couldn't overclock and even if you could over clock, the tiny vrm couldn't handle it. Thats probably why you can't overclock on A320's. They have weak power delivery. The m.2 on the A320's probably shares pcie lanes with a couple of your sata headers so you might end up having to choose between two sata drives or a nvme storage device but im totally guessing on that aspect.
I would choose the b350 for the decent vrm that is capable of handling an 8 core cpu no problem. Even if your only putting in that apu, the better power delivery will run cooler and...