In rehearsal, I’m asking this question when confronting close dissonances in adjacent parts: who should win?
The answer requires some knowledge of theory and analysis. If the parts are a whole- or half-step apart, one of them will likely be a chord tone, and the other probably a color note. Knowing which is which helps you understand how to balance it.
Then there’s the knowledge of vocal technique. Understanding the timbre of the voices in a particular range will affect the approach from each part. For example, a tenor on a D against an alto on an E; the tenor will be in a much more brilliant part of his range than the alto, and should adjust accordingly.
Finally, or maybe first of all, there should be an understanding of melody. The melody must speak most of all, so we must know who sings the melody.
Luckily, this comes down to a pretty easy rule. Most of the time, the upper note of a pair is the more important of the two. That note is far more likely to be the chord tone, it’s far more likely to be in a less-brilliant part of the range, and it’s more likely to be the melody.
Set that as a Factory Default Setting in your interpretation of close parts, and then do the analysis to make sure you’re right.