Depending on how the choir will be situated, ie. how they're sitting and being positioned, and the distance you have to work with, I would use either a 24L II or 35L on a full-frame. I prefer either of these over the 16-35L when doing group shots. The 14L will work but it will change the scaling quite a bit as it will take in more of the surroundings and reduces the individual faces. As AdamJ mentioned, you'll also need to frame the entire choir closer to the center line so as not to distort any individuals standing on either sides. You will also need a good, sturdy tripod and remote (or set the camera on timer).