Is it more efficient detect when audio clip ends with AudioSource.isPlaying or with AudioClip.length?

I want to trigger an event when an audio clip finishes playing. Would it be more CPU efficient to keep checking if the audio clip is playing with
WaitUntil(() => audioSource.isPlaying == false);

or would it be more efficient to use: WaitForSeconds(audioClip.length); to simply wait a certain amount of time?
How can run a test in Unity to see which one is more CPU efficient?