RDNA 2 and ampere performed pretty close on launch (in raster) and still do i.e. 3080 and 6800xt are neck and neck depending on title, 6800xt will win some, lose some, same goes for 6900xt vs 3090 and 3090ti vs 6950xt. SAM/rebar came along which offered a decent jump in some games compared to ampere i.e. ACV, forza 5, halo but then nvidia recently improved their dx 12/fine wined ampere and rebar performance to close the gap significantly in those main titles where amd had a decent lead.
I suspect there might be a good chance of some fine wine with this chiplet design but that's an unknown and if history is anything to go by.... by the time amd make those serious improvements, we'll be on the next lot of hardware again, of course if you buy to last years then it is a more valid reason but given you got a 3090 and now a 6900xt, this won't apply to you
Depends really how much of a difference there is, that and we need to see how well RDNA 3 will scale. But iirc, both are rated at the same wattage, that and we know the 4080 is a bit of a power efficiency beast as is:
Well both but ruling out dlss/fg, ampere is still ahead considerably in terms of performance per watt in RT especially when/if you factor in how well ampere undervolts e.g. my 3080 is using a 100w less than stock settings for same/better performance.