saintpit

X570 has more potential uogradability that b550 because it has pcie4 across the whole board (unless they cheap out) so it could last even longer.  The lowest end x570 boards are arguably worse than even mid grade b550 though.  The particulars of exactly how b550 does it’s pcie4&3 trick and how much faster gpu cards get are going to matter a lot as to whether and how easily and what kind of add on cards possible to put on to b550.  If you plan to change cpu and add cards though you may be better off buying a b450 and just doing a cpu/mobo swap when the time comes to whatever is new.  It’s one of those six of one half dozen of the other calls.

Try out the onboard first. If it's not good enough for you then upgrade. But don't get a add in card, get an external DAC and amp or DAC/amp combo unit. 

Ah, well, okay, if you have decided. If you don't mind, I guess you can view this video explaining more details of different B550 board:
 
This is from Gamers Nexus channel, and it actually talks about VRM and features the boards have. It's a bit lengthy, but provide a deep explanation on the board's limitation and note worthy features.

No consumer card today cam saturate gen 3 speed. Only the 2080ti just recently saturated gen 2. 

Yep.  If you put two cards in slots though it goes from pcie3x16 to pcie3x8.  3 cards and it’s x8, x4 and x4.  Four cards and it’s x4 all the way across.  It only occasionally matters.  It’s Why I said 99%.  Depends on if you consider a titan consumer or not.  Edge case.  May change in a few weeks though.
 
As a computer ages ports change and pcie cards often need to be added if upgrading rather than replacing.

Yeah, forgot to include that. With that said, B550 will have a long time support compare to B450.

No worries.  Depends on how the sentence is read. Clarification would be x570 over b450. Its persnickety though.

B550 is more recent than x570. Only by a few months though.

Only the first pcie slot?  Hmmm..  I may possibly be full of it.  Or rather it seems likely but I’m not sure in which way.
 
I’ve seen b550 cards claim SLI capacity.  I think.  The issue with pcie3 is while 99% of GPUs today can saturate only half of pcie3x16, this may change.  With pcie3 if you’ve got a really fast pcie3 card you can use only one of the available slots on a pcie3 motherboard.   Pcie4 is twice as fast as pcie3 though, making it again “more than you can possibly use in one slot”.  If they’re splitting them though it really matters how they are split if you want to use more than just one pcie card.  If it’s pcie4x16 or pcie4x8 + pcie3x16 it’s even better for multiple cards.  It might not be like that though. 
 
In either instance it means b550 has more upgrade capacity but costs more.  It might improve the use-it-now ability of b550 though because you could put more cards in it without slowing a fast gpu.

For a casual user. I really wouldn't think they would notice. The difference. Gen 3 is still plenty. 

I would highly recommend you to get X570 motherboard. X570 motherboards are more recent, so you should be able to expect manufacturer to support the board for another 2 or 3 years. And with modern IO such as PCIe Gen 4 will make it pretty much future proof if many manufacturer began to take advantage of PCIe Gen 4 (PCIe Gen 4 theoretically provide up to 1.96 GB/s or 15.75 Gb/s per lanes, capable to support 10GbE Ethernet or USB 3.2 Gen 2 Superspeed+ port using only PCIe x1 slot).
 
Anyway, I would go for B550 if I'm going to use 2 M2 drives, as the chipset can support Gen3 NVMe SSD and PCIe Gen3 add-in card, such as many modern WiFi AC card.
 
For B450, I guess it's still okay if you don't mind of slower PCIe Gen 2 speed.

Budget? 
B450 boards are dmi 3 all around. While b550 allows thr first x16 and the first m.2 to be gen 4 ( basically any lane that's connected to the cpu ). 
You're powering a 6 core. So a decent b450 board is more than enough. 
For b450. 
The tomahawk max 
Mortar max 
Pro 4. Pro vdh max. A pro max (non m ). 
For b550. 
Tomahawk mag. 
Mortar. 
Aorus pro. 
Strix f. 
And a cheap but decent b550 board. 
B550 pro 4. 

It’s all about use case and what the future holds.  Crystal balls don’t work very well.  

If you go from machine to machine rather than upgrading a single machine b450 makes more sense.  If you upgrade b550 does.
 
B550 has two architecture things: pcie4 and big VRM.  Big VRM means more potential for cpu upgrade in the future.  Pcie4 means potentially more use for larger numbers of  pcie cards.  IF all the cards are pcie 4 of which almost none are atm.  It also sometimes has USB 3.1 gen2 and thunderbolt3.  It’s got more potential future use assuming certain things happen.  It’s more expensive though and often provides no immediate advantage other than the ports.
 
UPDATE: seems underlined  bit is wrong.   Stupid phone won’t let me change it to strike through.