I tend (well... I really don't 'tend', but actually 'do') to agree with your choirmistress. The more talented ones should serve as section leaders for the entire choir, who should grow into the desired polyphony. From this, much can be learnt about blend and choral tone, and so on and so forth. I don't see your situation as 'polyphony or not polyphony' but as an entire choir growing in musicianship, repertory, and group elan.

I just might consider having the quartet do one and only one motet as an example to the others, with the message that this is where the entire choir are going. It would be carefully explained that the quartet are not stealing the choir's role but providing them with a one time sampling of what's to come. I would do this only if I thought it necessary as a means of eliciting enthusiasm from the entire group.

A quartet is not a choir (and I was a paid scholarship post grad singer and director of a five person schola at university.) A quartet can, of course, sing SATB choral works, but only emulate what a composer really heard.*
I don't subscribe necessarily to JMO's maxim of three to a part minimum, as two per in a decent aural environment can supply a worthy spectrum of timbre/tone for ambient sound. So, have the quartet augmented by a succession of worthy volunteers; and let the quartet soloists mentor the amateurs.
As I've said before, a solely paid ensemble cannot thrive long term in most parochial applications. Choral scholars should theoretically be motivated to be teachers as much as students at every stage of their under/grad careers.
*As a high school teacher I hosted a quintet from St Petersburg for a day. They sang the Rach. "Bogoraditze...." and I couldn't help but think "Well, this is a novelty and quaint." And it was just that.

The intricate web of relationships of singers of different levels and experience has been and will continue to be the downfall of many directors.

Singers who rise above the level of the group from within the group are accepted. Those who join the group are rejected - the more the talent, the more the aversion to them.

Mello's comment reminds me that I was encouraged by the old guard not to stay for the late Mass, only to find that four of the old-timers were attempting to rather poorly sing the anthems and motets from the earlier Mass I directed with the large choir on their own...discovered when one tenor complained to me about being begged to show up and sing bass at this later Mass when one of the four was out of town.

At the same time, a small group singing verses when the choir sings a polyphonic antiphon can be very effective and a good experience, since the choir will not have to learn the verses - a relief - and they will enjoy the poly part.

Webs work for spiders, don't they?

Wendy (where are you, dear?) did the Allegri Miserere the first year she was directing, how did you pull that off?

Small group <> several quartets. Small group = a smaller group than the full choir which typically spans SATB. Of a group of 16 (for example), it may be 6-8 singers, with some parts only 1 person.

SML mentioned the Allegri. There are the three sections for separate choirs. SSATB (full choir), SATB (small group - in that case 4-6 singers), chant verses (2 cantors). There is overlap between the segments, but chant verses not necessarily all men, nor the SATB section (with the high C) for the full choir.

Biebl Ave Maria - restricted to small group of 8 singers across seven parts with much more likelihood of success than the full group singing it.

This year, my small group will do the Lotti Miserere (eventually full group), Victoria Iesu Dulcis (eventually full group), Victoria O Vos Omnes #2 (eventually full group), and the Biebl (always a small group). The small group = 3 Sop, 2 Alt, 1 Ten, 1 Baritone (sometime tenor, sometime bass), and 1 Bass.

Obviously if I'm intending the other pieces for the full choir it isn't something that they CAN'T do... but it's a time management issue. I introduce the pieces this year, the full choir is exposed to them (everyone has the music), the core of more experienced singers learns it much more quickly then spending time on the full group and is available to reduce my ramp-up time when I introduce to the full choir down the road.

It's a system that works for me in the situation I'm currently in. I wouldn't argue that it works in every choir...

Some of you have offered your 2 cents. Unfortunately the minimum coinage I can offer is 5c which is worthless to you lot anyway.
But here is my 5 cents.

I like both of your viewpoints.
I love the idea of auditioning for the different motets but much like the “Aussie of the month” award given in schools around Australia there will always have to be a rotated “pupil of the week” award whereby someone who never gets the AoTM award is awarded PoTW without doing anything!

Much is to be said for choral part leaders because that’s why rehearsals are important for all, the more experienced/proficient still should attend for the sake of those in need of learning.
That being said I like the textural difference of having 4 solo voices, which often you get in mass settings that include a separate benedictus or two separate Agnus Dei movements.

The trick would be to make sure it doesn’t come across as ‘elitist’ even if the group are advanced. I think sometimes people go overboard with the segregating and give them something exclusive like over the top copes or a badge or place them in a highly visible place in the church. If you can just make it part of the whole then why not give it the occasional burl?

I’m in a situation where I run a professional Choir and deal with several amateur choirs. I’m very careful to make sure that the amateurs feel just as cared for as the professionals. One of the best things I ever did was to get my professionals to attend masses with the amateurs or concert performances and they actually came and enjoyed it! The amateur choirs are now much more receiving of the professional ring ins I bring in.

I reckon you can do both. Have the odd occasion, for example, a Choir camp or very special feast where you get your best auditioners to perform as a chamber ensemble. I wouldn’t do it week in week out though because it denies some major improvements that can be made for the whole Choir.

Also 5 cents given to the person who can name the animal on the Aussie 5c piece without searching the net for the answer.

Wallaby.

Platypus.

Crocodile.

Kanga.

Nicole Kidman.