The main issue is not choosing between those exposure formulas — it’s sky brightness. If streetlights or a bright moon are lighting the scene, the Milky Way will be overwhelmed and won’t stand out, regardless of small exposure differences.

For best results:

• shoot from a genuinely dark location
• avoid streetlights and other ambient light
• shoot when the moon is below the horizon, especially avoid a full moon
• use your lens wide open and keep shutter speed short enough to avoid star trailing

The various “500 rule,” “600 rule,” or sample ISO recommendations are only rough starting points for dark-sky conditions. A one-stop difference between formulas is much less important than whether the sky itself is dark enough.

So before worrying about ISO 3200 vs 6400, make sure you’re shooting under dark, moonless skies. Once you do that, use the widest aperture available, a shutter speed based on focal length to limit trailing, and then raise ISO as needed for your camera’s noise tolerance.