Not easy to tell you which to buy. Both cards you listed are good, but price wise it is difficult to say if you can get either in your area for under $800, as you have some strange sales Tax laws in the US.
Prices are falling, but not sure you would get the Rx6900 XT for under $800 at this time. More likely to be $900-$1,500.
Do not get an XFX card, they are significantly cheaper for a reason.
I have just seen that the RX6800XT is currently available on Amazon.com for under $800. A Sapphire, Gigabyte or Asus card should work just fine in macOS.
This one is less than $700.
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