Zig has the advantage that it can add the syntax sugar right into the language instead of using macro "workarounds", but I'd guess in the end both have similar lowlevel memory layout (a struct with an enum tag, followed by a payload "raw union") and generated code (e.g. it would be interesting to look at the compiler outputs, they should be very similar).
This "advantage" is also its greatest disadvantage. I can stick these macro definitions in any existing project and start incrementally rolling them out. Migrating something — even partially — to Zig is a much bigger task.
