I grew up in a traditional church. It's about 15 minutes outside of the heart of Birmingham, AL. We had a pretty active music ministry - adult choir, youth choir, childrens choir, handbell choir, pianist, organist, etc. The adult choir, robed in white & green, consisted of approximately 20-25 members on a given Sunday, with a few extra for the special events. When I came in to 8th grade, the adult choir director (who was also my high school choir director) invited me and two of my buddies to join the choir. My voice was just starting to change, but somehow I sang bass. Because I had already taken 9 years of piano lessons and had sung in a boy choir for 5, my ear was already very well-trained, and the men were happy to have me sing.
Our choir rehearsals consisted of choral anthems that were sung during the offering. That's all we worked on, and I enjoyed it. We usually had no idea what congregational hymns were going to be sung until approximately 15 minutes before the service started, but it really didn't seem to matter to anyone. We all enjoyed singing the hymns with the congregation, but the choral anthems were what we worked for every week.
When I was entering into 12th grade, I attended a different church. It was the complete opposite of the church I had been attending. The music was as contemporary as you can get. Hillsong and the Passion Movement were spreading like wildfire, and this church was the place to go if you wanted to hear that kind of music. Within two weeks of my first visit, I was playing keyboard/synth for the youth worship band and the adult worship team. We had bass, electric guitar, 2 acoustic guitars, drums, percussion, praise team, choir and a worship leader. All-in-all, there were about 40 people on stage each week. It was a big, upcoming church, and today has a sanctuary that seats approximately 1500.
Our rehearsals at the new church had a completely different focus. Not only did choir/band rehearse together, but we spent our time on the worship songs that the congregation would be participating in each week. I can hardly remember us spending any time at all on music to be sung while the offering plate was being passed around. Usually, it was some choir or church member singing to a track. The focus here was on the worship songs, not the choral anthem.
So you have 2 choirs with 2 different goals: one spent its time on choral anthems, the other on the worship songs...is one more important than the other? People have made cases for both (i.e. "anthems are more musically challenging," or "it's easier to worship when there aren't so many notes to learn"). But I'll ask this question first: what is the role of a choir in worship? At its core, a choir exists to help the congregation worship. A choir is (hopefully) a group of singers using melody, harmony, rhythm and text to guide the congregation through a creative art experience that God supernaturally uses to help us experience him. Music is universally understood, and God uses it to speak to us, to humble us, to draw and woo us, and to encourage us. It's powerful stuff. So a choir is assembled to help create this powerful thing called music. And that music is part of a worship service where the end result is God's glorification.
My answer to the other question: Both the worship songs and the choral anthem have good use in a worship service. Like I said, God uses music to draw us to himself. However, I think in order for the choral anthem to work, the choir has to help the congregation get started from the get-go. Our goal at St. Andrews is to put together a complete worship service from start to finish. If the choir has no idea what the worship songs are until Sunday morning, we'd be sight-reading, and from my experience, it's VERY DIFFICULT to worship God from the heart when you're on stage, under the lights, sight-reading. The mind is just too distracted by notes and unfamiliar lyrics. And if the choir doesn't know the worship songs, they won't be doing any favors to the congregation by sight-reading. In this case, helping the congregation worship means knowing the songs they'll be singing and being able to express them in worship. So when I'm rehearsing our choir, we learn the songs the congregation is going to sing first, then the we work the piece(s) the congregation will be listening to for the offertory. When the choir is familiar with the worship songs, even impacted by them, it keeps our heads from being buried in our folders and allows us the freedom to "get lost" in God's presence. If this happens, a choral offertory will simply be a continuation of praise and worship. If not, it's just a nice performance.