other thank pcie 4.0, there is not really any fundamental difference between them. the features of the board may be different though, so you need to decide if your current board has what you need. for me personally, i'd only upgrade if there was some new feature i needed. could be more usb ports, rgb headers, sata ports, or better power delivery and so on that you may find you need to add. pcie 4.0 is not enough of a reason to upgrade in my opinion. next upgrade down the line will be all 4.0 storage and much cheaper by then as well.
That depends on exactly which X370 MB you have.
Other than PCI-E 4.0 and better VRMS the B550 and X570 boards don't offer much IMO.
I have an Asus ROG Strix X470 that I will use when I eventually (1-2 years) upgrade to a Ryzen 5000 CPU.
It has every feature I need so I probably won't be buying B550 or X570 MB.