Whilst the Julia version currently beats Mojo, I fully expect both to approach basically the same performance with enough tinkering, and for that performance to be on par with C or Fortran. A more interesting question is which version is more elegant, ‘obvious’ and maintainable. (Deeply familiar with both, but money is on Julia).
IMO the reason Julia gets to be this fast is because of LLVM, and the guy who created LLVM is also the creator of Mojo so there is something to be said about that
LLVM helps with its own optimizations, but more and more of those optimizations are being moved to the Julia side nowadays (since the language has more semantic understanding of the code and can do better optimizations). I believe the main thing LLVM helps with is portability across many platforms, without having to write individual backends for each one.