The B450s tend to have a more robust feature set as well as natively supporting the 2nd Gen Ryzen processors out of the box. Whereas the A320/B350s typically require a BIOS update in order to support the newer processors.
The major difference that makes it more attractive is the B450 chipset allows over-clocking of the CPU so you'll find generaly better CPU VRM's with heatsinks on them. But not always... there are plenty of B450 boards with VRM's no better than A320 boards (generaly from Asus and Gigabyte) so you have to watch for that if you do intend to overclock.
B450 chipsets also have slightly more PCIe lanes to allow more PCIe slots, SATA ports for drives and/or more USB ports. To go with this mfr's will glam up B450 boards a bit more, with color schemes and RGB options to make them even more appealing.
And of course, X470 has the best choice of PCIe lanes and so have even more SATA and USB ports. They also have the VRM's needed for extreme overclocking... to the point of overkill, unless you plan on sub-ambient cooling of the CPU itself. Which makes it quite impractical for day to day use.
There are indeed lots of A320 boards still in the marketplace dating from way back in Gen 1 Ryzen. So be careful to get one that supports 2nd gen Ryzen and Ryzen with Vega CPU's.
The differences between A320 and B450 include overclocking, multi-GPU, VRM, PCIe Lanes. a320 chipset is an entry-end chipset while b450 is a mainstream choice for mid-range pc. More details can be found in this post. Hope it help. AMD/AM4 Chipsets Comparison: A320, B350, B450, X370, X470, X399 and X570