AMD has not made any promise that 5xx boards are going to be compatible with Zen4/Ryzen 5xxx. The boards will be Zen3/Ryzen 4xxx compatible, but maybe nothing beyond that. In fact they only said they will support AM4 until DDR 5 is out. It is more than likely regardless of what board you are going to get it will only support up to Ryzen 4xxx at most.
So really it comes down to what are you buying now and what features you need. For instance I use MSI b450 Tomahawk Max boards with my Ryzen 3600 and 3700x and have no need for the more expensive 5xx boards.
And to add to that, Zen 3 is only supposed to be 20% faster than the Ryzen 3000 series, which is a bit but is it really worth another $300? Not really. You should wait until either Zen 3 is out this year, which case you're at the end of Socket AM4, or wait until Zen 4 next year which should bring DDR5 and, possibly, PCIe 5 and USB4 support. So if you can wait, wait.
Plus the new boards and memory will be very pricey too.
Every time new revision to memory comes along the new format, DDR5 will be horribly expensive while DDR4 falls through the floor. When prices for DDR4 fall too much then old lines are converted but that is expensive. DDR4 will survive for at least 12-16 months after DDR5 is introduced mostly for those who figure more RAM is cheaper than a new DDR5 motherboard, CPU etc
Memory perhaps, motherboard likely not any more expensive than they are now and probably a bit cheaper as manufacturers figure out how to optimize their boards for PCIe 4, if not moving to PCIe 5 as the spec has been finalized for over a year, thereby cutting costs.
PCIe is more desperately needed in servers than some gamer really needs. My X570 is PCIe 4.0 but I am spinning my wheels as the Intel SSD I have does not even exceed PCIe 2.0 bandwidth yet.
Maybe someday Intel will have a product with more aggregate bandwidth and chips on both sides of the M.2 device instead of the single sided unit being offered. When I reviewed my SSD I criticised Intel for being single sided, double sided would allow for a 4TB unit instead of 2TB.
DDR5 is now being sampled for servers, gamers will be with DDR4 for quite some time to come.
DDR4 scales up to 128GB on my X570 and I have 32GB now which is overkill
B550 boards, the higher end ones atleast have been created with higher memory frequency targets. The Aorus Master has 5400Mhz as max fequency also the x570 revisions have updated memory frequency in anticipation of the new Zen release. Granted no one is really using these high frequency but they are a engineering difference in the boards.
I would buy the most board for the money.
what boards are you looking at? and at what price point?