I have miked choirs for recording and PA.
The answer to your question depends a lot on the room.
In a quiet room, I love bidirectional-pattern mics in a Blumlein configuration. I get a glorious sound from two AEA R84s.
But if you can hear the air conditioning, audience coughing, etc., I would go with small-diaphragm condensor mics. The SM81 is a good choice for many things, including choirs, but really, you can go from pretty-darned cheap to very expensive and get good results.
For recording, fewer mics is better in order to avoid phase problems. For PA purposes, you will probably need more than 1 pair in order to get a decent balance.
Of course, there are mics made just for choirs, but they are intended for installed systems.