The real question is unification vs bidir more than HM vs bidir.

Unification is simple, not very hard to implement and more powerful. Bidir gives better error messages and is more "predictable".

I personnaly lean strongly towards unification. I think you can get good enough error messages and what you lose with bidir is not worse it. But clearly the Rust core team disagreed. They clearly don't mind annotations.

Anyway, here is my non answer: it's a trade off.