I mean, Java has faster developer speed since it's much less complicated than Rust. But it will also use a lot more memory. That being said, I guess most of what happens in Lemmy is database queries anyway and that will go equally fast in Java.
Rust is a very fast language but for a web app like Lemmy it's probably not that important that it's rust underneath.
But I really dislike Java. Lots of issues with code only working on a specific JDK, and code being very ugly due to all classes and shit.