I would really want to buy nvidia over overpriced RTX 4090. Is it possible for you to run some standard benchmark like this[1]?
Here is a link to a tom's HARDWARE stable diffusion benchmark from January to get a rough idea on where various cards fit in for that use case:
https://www.tomshardware.com/news/stable-diffusion-gpu-bench...
In the article they show a performance comparison chart here:
https://cdn.mos.cms.futurecdn.net/iURJZGwQMZnVBqnocbkqPa-970...