It won't make any difference to BG3 as you can play maxed out graphics with something a lot more modest than the new graphics cards.
Having said that the reviews are out and the consensus is 3080 and 6800XT are equal to each other and 6800 is better than 3070 but costs a bit more. They are all excellent cards though. The caveat is real-time ray-tracing where NVidia crushes AMD in most case, often by a huge amount. Whether this will continue to be the case with consoles using AMD tech and devs thus optimising for that, no one knows. Can't buy either of them though as there's no stock, so it's all hypothetical.
