I do not know if JPEG XL is competitive with AVIF for very compressed images, because that is mostly a concern for those who want to host at a minimum cost Web pages with embedded images. I assume that AVIF must be better for that purpose.

On the other hand, AVIF cannot compete with JPEG XL for high-quality photographs, which is something much more important for me, because it is a format suitable for storing my own photographs and it is a format that would make me appreciate positively a Web site that would have beautiful images in this format.