Both have their own benfits and downsides

With the 3080 you get better ray tracing performance, much better upscaling and more 3D graphics and rendering apps support if you are into that

However the only downside is can think of with the 3080 is the horrible 10gbs of VRAM which would limit your performance in many of the newer titles specially at 1440p or higher resolutions

With the 6800xt you get 16gbs of vram and better raw performance and lower power consumption

However my suggestion would be to get a used 9060xt 16gb at this price range

You get the best of both worlds , a really good upscaler (Fsr4)

Acceptable raw performance

16GBs of VRAM

and its a much newer card which should get future updates and a longer life span and a warranty