Here's an actual relevant HIGH QUALITY comparison with ssimulara2 scores/settings/etc I saved from a thread a while back. Not anime but not a real life photograph, making it a decent middle-ground test for image codecs in general. It would appear at least from my limited research that AVIF used to be dogshit and JXL was somewhat better in terms of general compression efficiency taking both photographic and non-photographic content into account. However something change in the past few years where AVIF has achieved parity with JXL on average.

I have to stress that ON AVERAGE there doesn't seem to be a "winner" in terms of compression efficiency, using JXL or AVIF will probably get you good results. With that said AVIF seems to performs better with anime content and JXL performs better with photographic content. Anyone who tells you otherwise is fucking trolling. Where JXL is arguably better is that it launched with progressive decoding day #1 whereas AVIF only recently got that so if slow connections are a concern then JXL has an advantage there unless you explicitly go out of your way to make your AVIF images progressive. Which to be fair at file sizes of just a few hundred KBs doesn't really seem necessary.