An interesting question. I don't have a definitive academic answer, but it seems to me that tone viii is the obvious choice. For reason, the reciting pitch of tone viii is Do (C), which is the key of the piece, and the final cadential note is Sol, the dominant. This should, perhaps, carry a little more weight. Tone viii should perhaps be the wiser choice. It seems to me that tone vii strongly implies a-minor, which makes the alternate C-major polyphony a little jarring. Then, there are the verses in a G-major tonality, which would be even more jarring.
Mode VII would be a more useful pairing, given that “Laudate púeri” is in mode VII for Lenten and green Sundays in the old rite. I believe this edition is taken from “Musica Divina” (Proske), which intended to compile “useful” music like that.
I’m wondering if that “utilitarian” advantage would have been just as true for Anerio?
Musically, yes, the obvious choice would seem to be mode VIII.
Felipe and I are thinking on the same lines: the Sunday antiphon Sit nomen Dom. is mode vii; that for Common of an Apostle is Spiritus et animae in mode viii; Common of Confessors is Sacerdotes Dei or Beatus ille servus both in mode vii...
The only major concern is in which pitch you sing both the chant and the polyphonic work. It is usually best to have some sort of strong tonal relationship between the two, especially if the polyphony is tonal (as opposed to modal polyphony such as organum). Either way, keep the solfeggio relationships the same (as in SOL in the chant should be the same as SOL in the polyphony, etc.).
I'm surprised the tone isn't quoted more often in the motet. I know that doesn't help you, but I could see it being more flexible based on that. If you were using an Iberian setting, the mode VII would be pretty weird since the polyphony would almost certainly have a substantial quote from the psalm tone. I only found one really clear one in the tenor of the Anerio.
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