If you are a Logic user, AU is the only choice that matters. Logic does not support VST3 natively; it demands Audio Units. This exclusivity forces developers to prioritize the AU format, meaning Logic users often get the most stable, bug-free versions of third-party plugins. The integration is seamless—AU plugins handle MIDI routing and side-chaining with a level of polish that often feels clunky in other formats.
Many VST plugins are also available in AU format. Always check before buying. audio unit (au)