How big is the difference between a 3090 and a 4090 for LoRA training?

I'm looking to get a GPU for gaming and SD. I can get a used 3090 for 700 USD or a used 4090 for ~3000 USD.

Both have the same VRAM size, which I understand is the most important thing for SD. How big is the difference between them in terms of speed for common tasks like image generation and LoRA training? Which would you recommend given the price difference?

Also, are AMD GPUs still unable to run SD? So far I have not considered AMD GPUs due to this limitation.