When balancing a chord, it’s important to take into mind tessitura, dynamics, chord note function, vowels, and any number of other possible challenges. But there’s a particular issue that clarifies the challenge.
When you have two voices a half-step away as part of a chord, it’s important to make sure there are no winners. Within fairly narrow parameters, the two notes should sound like an evenly-balanced tug-of-war, with strength in equal opposition. If one voices overbalances the other, the grittiness of the dissonance will be lost, and moreover the singers will be pulled to the overpowering note.
Or, as I told my singers tonight, “When one voice wins, we all lose.”