How much do you care about raytracing? AMD are behind in this area but the 6900XT is still around 3070 performance in raytracing in modern games which isn't bad and it's sometimes faster than a 3090 in rasterisation (the vast majority of games are still made this way). It also has 60% more video memory than a 3080. Nvidia have DLSS which allows upscaling from a lower resolution to increase performance while trying to maintain as much image quality as possible, but now AMD have an equivalent called FSR which although not quite as good as DLSS it works on everything including Nvidia and is much easier to implement in games than DLSS so in future may become the default option for developers to use in their games. There are future versions of FSR planned with improvements, this is only version one and it's open source so can be improved by the community and any developer who uses it. It's not in many games yet but I expect this to change, the first major game to have it will be Far Cry 6.
With FSR we have to wait to see its full potential, DLSS is a more mature technology and in many more games at the moment.
If you can get a 6800XT I'd get that, it's virtually as fast as a 6900XT with same amount of RAM but is much cheaper. On the Nvidia side I prefer the 3080, not the TI, although the 10GB of VRAM may become a problem at 4K resolution with raytracing in the future.
You gain more performance from SAM/ReBar on an AMD GPU, up to 20% maximum vs 10% in very select circumstances.
AMD's driver control panel is now more modern and advanced than the Nvidia one which is the same as 5-10 years ago. The AMD one has built-in automatic benchmarking and performance stats for all your games, a performance and game recording in-game overlay and it's all very responsive and quick. When you install the driver I recommend selecting the standard profile not the gaming one as it enables things that can sometimes cause problems, if you want to enable those things you can do so later per game. If you don't want things like Radeon ReLive (the Shadowplay equivalent that allows you to record your gameplay then you can do a minimal install and you don't have to sign into an account like with Geforce Experience). I haven't used AMD for over a year so things might have changed again.